feat(coverage): pattern-library coverage measurement (#2692, milestone 288 step 7)

Server-side shape enumeration per bound repo — one archive download via the
forge adapter, definitions extracted with a Python mirror of the write-path
hook's awk rules (shared test vectors pin the two together) — compared
against recorded snippet locations by path+symbol. Summary is cached in the
settings KV with a freshness stamp; recomputed on webhook push (spawned off
the delivery path) or explicit refresh, never in a request path.

Surfaces: GET/POST /api/projects/<id>/coverage[/refresh], a project-page
card (estimate-labeled, largest-gaps chips), and a one-line evidence-carrying
entry in enter_project read from cache only.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

src/scribe/mcp/tools/projects.py

@@ -17,6 +17,7 @@ keeps working.
 from __future__ import annotations
 
 from scribe.mcp._context import current_user_id
+from scribe.services import coverage as coverage_svc
 from scribe.services import design_systems as design_systems_svc
 from scribe.services import milestones as milestones_svc
 from scribe.services import notes as notes_svc
@@ -56,7 +57,13 @@ async def enter_project(project_id: int) -> dict:
 
     Returns a dict with keys: project, milestone_summary, applicable_rules,
     project_rules, subscribed_rulebooks, applicable_rules_truncated,
-    open_tasks, recent_notes, design_system, systems.
+    open_tasks, recent_notes, design_system, systems, pattern_coverage.
+
+    `pattern_coverage` (usually null) is a one-line estimate of how much of
+    the bound repo's code has recorded snippets — e.g. "pattern-library
+    coverage: 34/210 shapes recorded (estimate); largest gaps: internal/api".
+    When present, treat the gaps as a standing invitation: as you touch code
+    in those areas, record the shapes you find with create_snippet.
 
     `systems` is the project's vocabulary of named subsystems/areas. It is
     returned here so you can TAG as you write: when creating or meaningfully
@@ -128,8 +135,17 @@ async def enter_project(project_id: int) -> dict:
             uid, project.design_system_id,
         )
 
+    # Cache read ONLY — computing coverage moves a repo tarball and never
+    # belongs in this request path. Null is the ordinary state (no forge, or
+    # never computed); the line appears exactly when there is evidence. Read
+    # on the OWNER's id: bindings and the cache live with the project owner.
+    coverage = await coverage_svc.cached_coverage(
+        project.user_id or uid, project_id
+    )
+
     return {
         "project": project.to_dict(),
+        "pattern_coverage": coverage_svc.coverage_line(coverage) if coverage else None,
         # Trimmed to what tagging needs. The full charter is get_system's job —
         # this list rides along on every session start, so it stays lean.
         "systems": [