docs: name the reference skills other skills lean on

codebase-design, domain-modeling and grilling are pulled out as
standalone, model-invoked skills precisely so other skills can point
at them and so you can reach them as a reference independently of the
steps any wrapper mandates. Add a "Pulled out on purpose" section to
each that names who leans on it and why it stands alone, and slim the
overlapping "Where it fits" prose to avoid duplication.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

docs/engineering/codebase-design.md

@@ -14,7 +14,7 @@ npx skills update codebase-design
 
 `codebase-design` gives you a shared, precise vocabulary for designing **deep modules** — a lot of behaviour hidden behind a small interface, placed at a clean seam, testable through that interface.
 
-The load-bearing constraint: it is a **language, not a procedure**. It doesn't restructure your code or hand you a refactor plan — it fixes the words (module, interface, depth, seam, adapter, leverage, locality) so that every design conversation and every other skill that touches design speaks the same way. Consistent language is the whole point; "component," "service," "API," and "boundary" are deliberately banned because they blur the distinctions that matter.
+It is a **language, not a procedure**. It doesn't restructure your code or hand you a refactor plan — it fixes the words (module, interface, depth, seam, adapter, leverage, locality) so that every design conversation and every other skill that touches design speaks the same way. Consistent language is the whole point; "component," "service," "API," and "boundary" are deliberately banned because they blur the distinctions that matter.
 
 ## When to reach for it
 
@@ -32,6 +32,12 @@ Two checks do most of the work. The **deletion test**: imagine deleting the modu
 
 Callers and tests cross the same seam, so a well-placed interface gives tests something durable to aim at while the code underneath moves freely. That's why the vocabulary insists on **seam** (Feathers' term — a place you can change behaviour without editing there) over the overloaded "boundary," and why "interface" here means *every fact a caller must know*: signatures, yes, but also invariants, ordering, error modes, and performance — not just the type-level surface.
 
+## Pulled out on purpose
+
+`codebase-design` is the **single source of truth** for the deep-module vocabulary, split out as its own model-invoked skill so anything can reach it. Other skills point at it rather than restating the words: [tdd](https://aihero.dev/skills-tdd) borrows it to place a seam before writing the test, [improve-codebase-architecture](https://aihero.dev/skills-improve-codebase-architecture) leans on it while restructuring existing code, and [to-prd](https://aihero.dev/skills-to-prd) speaks it when it sketches seams and deepening opportunities before writing a spec.
+
+The point of keeping it standalone is that you can also reach for it on its own — as a **reference** for how to think about module design — without triggering the larger process any of those skills mandate. Fix the words once, in one place, and every design conversation inherits them.
+
 ## Where it fits
 
-`codebase-design` is a **reach-for-it-anytime standalone** and the shared vocabulary layer under the engineering skills — it's the language that [to-prd](https://aihero.dev/skills-to-prd) uses when it sketches seams and deepening opportunities before writing a spec, and that [improve-codebase-architecture](https://aihero.dev/skills-improve-codebase-architecture) leans on when it restructures existing code. Its closest neighbour is [domain-modeling](https://aihero.dev/skills-domain-modeling), the parallel vocabulary skill for the problem domain rather than the module structure. When you're unsure which skill or flow fits, [ask-matt](https://aihero.dev/skills-ask-matt) routes you.
+`codebase-design` is a **reach-for-it-anytime standalone** — the shared vocabulary layer under the engineering skills. Its closest neighbour is [domain-modeling](https://aihero.dev/skills-domain-modeling), the parallel vocabulary skill for the problem domain rather than the module structure. When you're unsure which skill or flow fits, [ask-matt](https://aihero.dev/skills-ask-matt) routes you.

docs/engineering/domain-modeling.md

@@ -14,7 +14,7 @@ npx skills update domain-modeling
 
 `domain-modeling` builds and sharpens a project's **ubiquitous language** as you design — challenging fuzzy terms, stress-testing relationships with concrete scenarios, and writing the glossary and decisions down the moment they crystallise.
 
-The load-bearing constraint: this is the **active** discipline, not the passive one. Merely reading `CONTEXT.md` to borrow its vocabulary is a one-line habit any skill can do; this skill is for when you are *changing* the model — coining a canonical term, catching a contradiction between the code and what you just said, recording a hard-to-reverse decision. And it keeps the glossary clean: `CONTEXT.md` is a glossary and nothing else — no implementation details, no spec, no scratch pad.
+This is the **active** discipline, not the passive one. Merely reading `CONTEXT.md` to borrow its vocabulary is a one-line habit any skill can do; this skill is for when you are *changing* the model — coining a canonical term, catching a contradiction between the code and what you just said, recording a hard-to-reverse decision. And it keeps the glossary clean: `CONTEXT.md` is a glossary and nothing else — no implementation details, no spec, no scratch pad.
 
 ## When to reach for it
 
@@ -35,6 +35,12 @@ Two artifacts, two different bars:
 
 The move that makes it click: when you state how something works, the skill cross-references the code and surfaces the contradiction — "your code cancels entire Orders, but you just said partial cancellation is possible — which is right?" The language and the code are forced to agree.
 
+## Pulled out on purpose
+
+`domain-modeling` is the **single source of truth** for building the project's ubiquitous language, split out as its own model-invoked skill so any other skill can reach it. [grill-with-docs](https://aihero.dev/skills-grill-with-docs) leans on it to record terms and decisions as a grilling session runs, [triage](https://aihero.dev/skills-triage) uses it to keep tickets in the project's own words, and [improve-codebase-architecture](https://aihero.dev/skills-improve-codebase-architecture) reaches for it while it works.
+
+Keeping it standalone means you can also reach for it directly — as a **reference** for how to sharpen a model — without committing to the steps any of those skills mandate. The language lives in one place, and everything that needs it points there.
+
 ## Where it fits
 
-`domain-modeling` is a reach-for-it-anytime standalone that maintains the vocabulary the rest of the work depends on — so it often runs *underneath* other skills rather than at a fixed step. Its closest neighbour is [codebase-design](https://aihero.dev/skills-codebase-design), because a shared language is what lets you name a deep module and its seam precisely; downstream, a settled glossary is exactly what [to-prd](https://aihero.dev/skills-to-prd) synthesises into a spec written in the project's own words. When you're unsure which skill or flow fits, [ask-matt](https://aihero.dev/skills-ask-matt) routes you.
+`domain-modeling` is a **reach-for-it-anytime standalone** that runs *underneath* other skills as often as at a fixed step. Its closest neighbour is [codebase-design](https://aihero.dev/skills-codebase-design), because a shared language is what lets you name a deep module and its seam precisely; downstream, a settled glossary is exactly what [to-prd](https://aihero.dev/skills-to-prd) synthesises into a spec written in the project's own words. When you're unsure which skill or flow fits, [ask-matt](https://aihero.dev/skills-ask-matt) routes you.