docs: end the build chain at implement, not tdd

The canonical build chain is grill-with-docs → to-prd → to-issues →
implement, with tdd as the red-green loop implement drives internally
rather than a chain step of its own.

- Update the chain diagram on grill-with-docs, to-prd, to-issues, and tdd.
- Reframe tdd's "Where it fits": it's the engine inside the implement
  step, still independently reachable, no longer the terminal.
- Point to-prd and to-issues at implement as the thing that builds the
  tickets, giving implement the inbound links it previously lacked.

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

docs/engineering/grill-with-docs.md

@@ -44,7 +44,7 @@ What makes this variant its own skill is where the answers go. As the grill runs
 `grill-with-docs` is the opening step of the main build chain:
 
 ```txt
-grill-with-docs → to-prd → to-issues → tdd
+grill-with-docs → to-prd → to-issues → implement
 ```
 
 It comes first, before anything is written down as a spec: it produces the shared understanding and settled vocabulary that [to-prd](https://aihero.dev/skills-to-prd) then synthesises into a PRD without re-interviewing you. Its close neighbours are [grilling](https://aihero.dev/skills-grilling), the same interview without the docs, and [domain-modeling](https://aihero.dev/skills-domain-modeling), the glossary-and-ADR discipline it drives. When you're unsure which skill or flow fits, [ask-matt](https://aihero.dev/skills-ask-matt) routes you.

docs/engineering/tdd.md

@@ -38,10 +38,10 @@ Refactoring only happens once the suite is green; never while red.
 
 ## Where it fits
 
-`tdd` is the final step of the main build chain:
+`tdd` is the red-green loop the main build chain runs to write code:
 
 ```txt
-grill-with-docs → to-prd → to-issues → tdd
+grill-with-docs → to-prd → to-issues → implement
 ```
 
-It implements the tickets the earlier steps produced. Its key neighbours are [to-issues](https://aihero.dev/skills-to-issues), which hands it an independently-grabbable issue to build, and [codebase-design](https://aihero.dev/skills-codebase-design), which it leans on to find deep-module seams worth testing at. When you're unsure which skill or flow fits, [ask-matt](https://aihero.dev/skills-ask-matt) routes you.
+[implement](https://aihero.dev/skills-implement) is the chain's final step, and it drives `tdd` internally to build each ticket test-first — so `tdd` is the engine inside that step rather than a step of its own. You can also reach for it directly, whenever there's a concrete behaviour to build without a full spec. Its other neighbour is [codebase-design](https://aihero.dev/skills-codebase-design), which it leans on to find deep-module seams worth testing at. When you're unsure which skill or flow fits, [ask-matt](https://aihero.dev/skills-ask-matt) routes you.

docs/engineering/to-issues.md

@@ -37,7 +37,7 @@ Before slicing, `to-issues` looks for prefactoring — "make the change easy, th
 `to-issues` is a step in the main build chain:
 
 ```txt
-grill-with-docs → to-prd → to-issues → tdd
+grill-with-docs → to-prd → to-issues → implement
 ```
 
-It sits between [to-prd](https://aihero.dev/skills-to-prd), which hands it a settled spec with user stories to slice against, and [tdd](https://aihero.dev/skills-tdd), which implements each independently-grabbable issue test-first. When you're unsure which skill or flow fits, [ask-matt](https://aihero.dev/skills-ask-matt) routes you.
+It sits between [to-prd](https://aihero.dev/skills-to-prd), which hands it a settled spec with user stories to slice against, and [implement](https://aihero.dev/skills-implement), which builds each independently-grabbable issue, driving [tdd](https://aihero.dev/skills-tdd) internally to write the tests test-first. When you're unsure which skill or flow fits, [ask-matt](https://aihero.dev/skills-ask-matt) routes you.

docs/engineering/to-prd.md

@@ -53,7 +53,7 @@ That matters for agentic development: a good interface gives tests something dur
 `to-prd` is a step in the main build chain:
 
 ```txt
-grill-with-docs → to-prd → to-issues → tdd
+grill-with-docs → to-prd → to-issues → implement
 ```
 
-Reach for it after the plan and domain language are resolved, and before you break the work into implementation tickets. Its key neighbours are [grill-with-docs](https://aihero.dev/skills-grill-with-docs), which sharpens the context so the PRD is precise, and [to-issues](https://aihero.dev/skills-to-issues), which turns the PRD into independently-grabbable issues for [tdd](https://aihero.dev/skills-tdd) to implement. When you're unsure which skill or flow fits, [ask-matt](https://aihero.dev/skills-ask-matt) routes you.
+Reach for it after the plan and domain language are resolved, and before you break the work into implementation tickets. Its key neighbours are [grill-with-docs](https://aihero.dev/skills-grill-with-docs), which sharpens the context so the PRD is precise, and [to-issues](https://aihero.dev/skills-to-issues), which turns the PRD into independently-grabbable issues for [implement](https://aihero.dev/skills-implement) to build. When you're unsure which skill or flow fits, [ask-matt](https://aihero.dev/skills-ask-matt) routes you.