feat(scribe): snippet recording nudge — reusing-code skill + MCP/SessionStart guidance

Step 5 of the Drafter recall milestone (#227): teach agents the two
snippet reflexes — search recorded snippets before writing a new
helper/util/component, and record something reusable the moment it's
built — via the app's own instruction surfaces, not a Scribe rule
(project rule #119). All instance-agnostic (rule #115).

- plugin/skills/reusing-code/SKILL.md: new auto-surfacing process-skill
  covering both reflexes (recall-before-rebuild + record-when-reusable).
- src/scribe/mcp/server.py: a Snippets paragraph in the MCP _INSTRUCTIONS.
- plugin/hooks/scribe_static_context.md: a "reuse before rebuilding"
  bullet in the SessionStart static context.
- plugin/.claude-plugin/plugin.json: version 0.1.12 -> 0.1.13 in the same
  change so the autoUpdate marketplace ships it (the #1040 lesson);
  description skill list updated.
- plugin/README.md: trued the process-skill list to what actually ships.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Pa2EsuB54BuWQ8GfJq9c7t

plugin/skills/reusing-code/SKILL.md

@@ -0,0 +1,45 @@
+---
+name: reusing-code
+description: Use when you're about to write a helper, utility, hook, or reusable component — search recorded snippets FIRST so prior art is reused instead of re-solved. And the moment you build or notice something reusable, record it as a snippet so a later session finds it. Triggers on "write a util/helper", "I need a function that…", "let me add a component", or just having built something worth reusing.
+---
+
+# Reusing code — recall before you rebuild
+
+Reusable code is worth writing once. Scribe stores **snippets** — a named,
+reusable function or component recorded with its language, signature, canonical
+location (repo · path · symbol), a one-line *"when to reach for it,"* and the
+code itself — so prior art can surface *before* it's re-written as a one-off.
+Snippets are ordinary embedded notes, so a recorded one also surfaces on its own
+through recall/auto-inject; this skill is the active reflex around that.
+
+## Before you write a new helper — search first
+
+- About to write a utility, hook, formatter, adapter, or a reusable component?
+  **Search snippets before writing it.** `list_snippets(q="…")` (or a plain
+  `search`) — a matching one may already exist, in this project or another.
+- If a snippet fits, pull it in full with `get_snippet(id)` and reuse it — its
+  `location` points at the reference implementation. Adapt, don't re-derive.
+- If auto-inject already surfaced a snippet title that looks relevant, that's
+  your cue to `get_snippet` it rather than start from scratch.
+
+## The moment you build something reusable — record it
+
+- Just wrote (or noticed) a helper, hook, pattern, or component worth repeating?
+  Record it with `create_snippet` while it's fresh:
+  - **name** — what it's called, e.g. `useDebouncedRef`.
+  - **code** — the implementation.
+  - **when_to_use** — one sharp line on when to reach for it. This becomes part
+    of the title, so it's what a later recall menu shows — make it earn the pull.
+  - **language**, **signature**, and **location** (`repo` / `path` / `symbol`)
+    so the recorded copy points back at the canonical source.
+  - **project_id** / **system_ids** to associate it with the work it belongs to.
+- Record the *reference* implementation, not every call site — one good entry
+  per reusable thing. If it already exists, `update_snippet` it instead of
+  recording a second copy (the create gate will flag a near-duplicate anyway).
+
+## Why this pays off
+
+A one-off written a second time is the cost this avoids. Recording a snippet
+once — with a location and a crisp "when to use" — means the next session is
+offered the prior art instead of re-solving it. Search before writing; record
+what's worth reusing.