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/README.md

@@ -7,8 +7,9 @@ instance into a first-class Claude Code extension:
   rulebook (the `scribe` server).
 - **Session-start push channel** — a `SessionStart` hook injects your always-on
   rules + active-project context so Scribe surfaces *without being asked*.
-- **Universal process-skills** — brainstorm, systematic-debugging, TDD,
-  writing-plans, verification, receiving-code-review (replaces superpowers).
+- **Universal process-skills** — using-scribe, writing-plans,
+  systematic-debugging, verification, brainstorming, reusing-code (record and
+  recall reusable code as snippets). Replaces superpowers.
 - **Your Scribe Processes as skills** — saved Processes are synced into local
   `~/.claude/skills/scribe-proc-*` stubs that auto-surface by relevance; the
   stub fetches the live procedure via `get_process`. Refreshed each session and