docs: add human-facing pages for the remaining 22 promoted skills

Adds a docs page for every promoted skill that lacked one, following
.agents/writing-docs.md and using docs/engineering/to-prd.md as the
worked exemplar. Covers all of engineering/ (bar to-prd, already done),
productivity/, and misc/.

Each page states its load-bearing constraint, its invocation mode and
trigger boundary, surfaces the skill's leading word, and routes back to
ask-matt so the set forms a connected router with no dead ends.

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

docs/productivity/writing-great-skills.md

@@ -0,0 +1,45 @@
+Quickstart:
+
+```bash
+npx skills add mattpocock/skills --skill=writing-great-skills
+```
+
+```bash
+npx skills update writing-great-skills
+```
+
+[Source](https://github.com/mattpocock/skills/tree/main/skills/productivity/writing-great-skills)
+
+## What it does
+
+`writing-great-skills` is the reference you write and edit skills against — the shared vocabulary and principles that make a skill predictable.
+
+The load-bearing constraint: a skill's job is to wrangle determinism out of a stochastic system, so the goal is not the same *output* every run but the same *process*. **Predictability** is the root virtue, and every design choice is judged against it — not against how clever, complete, or exhaustive the skill reads.
+
+## When to reach for it
+
+You invoke this by typing `/writing-great-skills` — the agent won't reach for it on its own.
+
+Reach for it whenever you're authoring a new skill or editing an existing one and want it to behave the same way every time: deciding invocation mode, writing a description, choosing what lives in `SKILL.md` versus a linked file, or diagnosing why a skill misfires.
+
+## Cognitive load
+
+The concept the whole reference turns on is **cognitive load** — and its counterpart, **context load**. Every skill spends one or the other:
+
+- A **model-invoked** skill keeps a description in the window every turn, so it costs **context load** but fires on its own.
+- A **user-invoked** skill strips that description; it costs zero context load, but now *you* are the index that has to remember it exists — that's **cognitive load**.
+
+Most of these skills are user-invoked, which is why cognitive load is the pressure the whole system is built to manage: when user-invoked skills multiply past what you can hold in your head, the cure is a **router skill** that names the others and when to reach for each. Once you're thinking in these two loads, most authoring decisions — split or don't, inline or disclose, model- or user-invoked — become the same trade made in different places.
+
+## The other levers
+
+The rest of the reference is the toolkit for spending those loads well:
+
+- **Leading words** — a compact concept already in the model's pretraining (_tight_, _red_, _tracer bullet_) that the agent thinks with while running the skill. It anchors execution *and* invocation in the fewest tokens; hunt restatements that a single word can retire.
+- **Information hierarchy** — the ladder from in-skill step, to in-skill reference, to external reference behind a **context pointer**. **Progressive disclosure** is the move down that ladder so the top stays legible.
+- **Pruning** — single source of truth, relevance, and the no-op test applied sentence by sentence, against **sediment** and **sprawl**.
+- **Failure modes** — **premature completion**, **duplication**, **sediment**, **sprawl**, **no-op** — to diagnose a skill that isn't behaving.
+
+## Where it fits
+
+This is a reach-for-it-anytime standalone reference — the meta-skill you consult while building the rest of the set, not a step in a chain. Its natural neighbour is any router you maintain, because a router is the direct cure for the cognitive load that user-invoked skills pile up; when you're unsure which skill or flow fits a task, [ask-matt](https://aihero.dev/skills-ask-matt) routes you over the whole set.