feat(plugin): ship the Scribe Claude Code plugin in-repo (marketplace + plugin)

Per operator: the plugin lives in the app repo so it ships and versions in
lockstep with the app and the /api/plugin/context contract it targets (same
co-location rationale as the former in-repo MCP). A git-cloned marketplace
supports relative plugin sources, so the FabledScribe repo IS the marketplace.

- .claude-plugin/marketplace.json — source ./plugin
- plugin/.claude-plugin/plugin.json — userConfig (base URL, api key, project id)
- plugin/.mcp.json — http scribe server, ${user_config.*} substitution
- plugin/hooks/ — SessionStart push-channel hook (fail-open)
- plugin/skills/using-scribe — bootstrap skill
- plugin/README.md — install via the FabledScribe repo marketplace

Phase 2 of plan #755. Install/userConfig-substitution test pending.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

plugin/skills/using-scribe/SKILL.md

@@ -0,0 +1,43 @@
+---
+name: using-scribe
+description: Use at the start of any session and before answering questions about the operator's work or starting a task — establishes the Scribe-first reflex (recall before acting, load standing rules, update over duplicate, plan in Scribe not in files).
+---
+
+# Using Scribe
+
+Scribe is the operator's self-hosted second brain (notes, tasks, projects,
+milestones, events, typed entities) and rulebook, reachable through the bundled
+`scribe` MCP server. Its value is mostly in what it **already holds** — so make
+reading it a reflex, not something you wait to be asked for.
+
+## The reflex
+
+1. **Recall before acting.** Before answering a question about the operator's
+   work, or starting a task, `search` Scribe (and `list_tasks` / `list_notes`)
+   for prior art — an existing ticket, decision, or dev-log — instead of
+   re-deriving it or opening a duplicate. When a project is in scope, pass its
+   `project_id` so results stay scoped.
+
+2. **Standing rules are binding.** The SessionStart context lists the operator's
+   always-on rule titles. Treat them as binding. Pull full text with
+   `list_always_on_rules()` or `get_rule(id)` when a rule is about to bite.
+   When a project is in scope, `enter_project(id)` also returns its applicable
+   rules.
+
+3. **Update over duplicate.** When recording, prefer updating an existing
+   note/rule/task over creating a new one. Search first; revise what's there.
+
+4. **Plans live in Scribe.** For non-trivial work call `start_planning(project_id,
+   title)` FIRST — the plan body + step checklist live in the `kind=plan` task,
+   progress goes in work-logs (`add_task_log`). Do not write plans/specs to local
+   `.md` files.
+
+5. **Keep state honest.** Set a task `in_progress` when you start it, `done` the
+   moment it's complete; log progress as you go.
+
+## Other Scribe process-skills
+
+This plugin also ships focused process-skills — brainstorming, systematic
+debugging, test-driven development, writing-plans, verification, receiving code
+review. Reach for the matching one when its situation arises, the same way you
+reach for this skill.