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/hooks/hooks.json

@@ -0,0 +1,14 @@
+{
+  "hooks": {
+    "SessionStart": [
+      {
+        "hooks": [
+          {
+            "type": "command",
+            "command": "SCRIBE_URL=\"${user_config.api_endpoint}\" SCRIBE_TOKEN=\"${user_config.api_token}\" SCRIBE_PROJECT_ID=\"${user_config.project_id}\" bash \"${CLAUDE_PLUGIN_ROOT}/hooks/scribe_session_context.sh\""
+          }
+        ]
+      }
+    ]
+  }
+}