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/handoff.md

@@ -0,0 +1,36 @@
+Quickstart:
+
+```bash
+npx skills add mattpocock/skills --skill=handoff
+```
+
+```bash
+npx skills update handoff
+```
+
+[Source](https://github.com/mattpocock/skills/tree/main/skills/productivity/handoff)
+
+## What it does
+
+`handoff` compacts the current conversation into a **handoff document** — a single write-up a fresh agent can read to pick up the work where you left off.
+
+The load-bearing constraint: it does **not** re-state what already lives elsewhere. Anything captured in a PRD, plan, ADR, issue, commit, or diff is referenced by path or URL, never copied. The document carries only the live thread — what you were doing, why, and what's next — and it's saved to your OS's temporary directory, not into the workspace, so it never becomes another artifact to maintain.
+
+## When to reach for it
+
+You invoke this by typing `/handoff` — the agent won't reach for it on its own. Pass a note about what the next session is for and the document is tailored to it.
+
+Reach for this when a conversation has gone long enough that its context is at risk — you're near a context limit, wrapping for the day, or deliberately handing the work to another agent — and you want the thread preserved without dragging the whole transcript along.
+
+## What the document carries
+
+- **The live thread** — what's in flight and why, in the conversation's own terms, minus anything already written down elsewhere.
+- **Suggested skills** — a pointer to the skills the next agent should reach for to continue.
+- **References, not copies** — links and paths to the PRDs, plans, ADRs, issues, and diffs that hold the settled detail.
+- **Redacted secrets** — API keys, passwords, and PII stripped before the document is written.
+
+The idea to hold onto is **compaction**: a handoff is the conversation squeezed down to just its resumable core, so a fresh agent inherits the momentum, not the noise.
+
+## Where it fits
+
+`handoff` is a reach-for-it-anytime standalone — it sits at the seam between two sessions rather than inside a build chain. It pairs naturally with the artifact-producing skills whose output it points at: [to-prd](https://aihero.dev/skills-to-prd), because a finished PRD is exactly the kind of settled detail a handoff references instead of repeating. When you're unsure which skill fits the moment, [ask-matt](https://aihero.dev/skills-ask-matt) routes you.