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>

.claude-plugin/marketplace.json

@@ -0,0 +1,12 @@
+{
+  "name": "scribe-plugin",
+  "owner": { "name": "Bryan Van Deusen" },
+  "description": "Scribe ships its own Claude Code plugin from this repo, versioned in lockstep with the app + the /api/plugin/context contract.",
+  "plugins": [
+    {
+      "name": "scribe",
+      "source": "./plugin",
+      "description": "Scribe second brain: MCP tools + session-start push channel + universal process-skills."
+    }
+  ]
+}