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/misc/scaffold-exercises.md

@@ -0,0 +1,43 @@
+Quickstart:
+
+```bash
+npx skills add mattpocock/skills --skill=scaffold-exercises
+```
+
+```bash
+npx skills update scaffold-exercises
+```
+
+[Source](https://github.com/mattpocock/skills/tree/main/skills/misc/scaffold-exercises)
+
+## What it does
+
+`scaffold-exercises` reads a course plan and builds the exercise directory tree it describes — numbered sections, numbered exercises, and their `problem/`, `solution/`, and `explainer/` variant folders, each seeded with a stub `readme.md`.
+
+The load-bearing constraint: the scaffold is only done when it **passes `pnpm ai-hero-cli internal lint`**. The linter is the contract — folder shape, numeric prefixes, non-empty readmes, no stray `.gitkeep` or `speaker-notes.md`, no broken links. The skill scaffolds, runs the lint, and iterates until it's green rather than leaving you a tree that merely looks right.
+
+## When to reach for it
+
+Type `/scaffold-exercises`, or the agent reaches for it automatically when a task fits — turning a plan into stubbed exercise directories, or setting up a new course section.
+
+Reach for it when you have a plan (section names, exercise names, which variants each needs) and want the skeleton on disk and lint-clean before you write any real content. It only lays down structure and empty-but-valid readmes; the teaching material comes later.
+
+## Prerequisites
+
+The skill writes into an `exercises/` directory and validates with `pnpm ai-hero-cli internal lint`, so run it inside a course repo that has the AI Hero CLI available. Outside that tooling the lint step — the whole point — can't run.
+
+## Lint-passing is the spec
+
+The naming isn't cosmetic: sections are `XX-section-name/`, exercises are `XX.YY-exercise-name/` in dash-case, and every variant folder carries a non-empty `readme.md`. Those rules exist because the linter enforces them, and the linter is what downstream course tooling relies on. So the mental model is inverted from "make a folder tree" to "make a tree the linter accepts" — default new stubs to `explainer/`, keep readmes real (a single title line counts), and let the lint pass be the definition of done.
+
+Renumbering later follows the same rule: use `git mv` so history survives, then re-run lint.
+
+## It's working if
+
+- `pnpm ai-hero-cli internal lint` passes with the new tree in place.
+- Every variant folder has a non-empty `readme.md`; no `.gitkeep` or `speaker-notes.md` slipped in.
+- Section and exercise folders carry correct `XX` / `XX.YY` prefixes in dash-case.
+
+## Where it fits
+
+`scaffold-exercises` is a reach-for-it-anytime standalone at the start of building a course section: it produces the empty, lint-clean scaffold you then fill in. It sits next to Matt's course-planning work — once a plan exists, this turns it into directories — but it owns no planning itself. When you're unsure which skill or flow fits, [ask-matt](https://aihero.dev/skills-ask-matt) routes you.