feat(systems): read-side teeth — the vocabulary at session start, a search filter, and the state/chronicle instructions

Step 4 of #278, product half. The audit that motivated it: one System in
project 2, thirty records tagged, nothing since July 28 — three days after the
feature landed. Not a discipline failure; retrieval was completely blind to
the association (zero references in embeddings, knowledge, search, auto-inject,
or enter_project), so tagging was a write-side label with no read-side payoff,
and labels nobody reads don't get maintained.

Three changes, ordered by what makes the others workable:

1. enter_project returns the project's Systems (id, name, first line of the
   charter). Load-bearing for the tagging instruction: you cannot ask an agent
   to check a record against a vocabulary it never sees. Trimmed because it
   rides on every session start; the full charter stays get_system's job.
   Present-and-empty rather than absent when a project has none — "no named
   areas yet" is information the create-the-System instruction acts on.

2. search accepts system_id, MCP and REST (#33). Implemented once in
   semantic_search_notes as an EXISTS against record_systems — an association
   filter deciding candidate-set membership before scoring, like project_id,
   not a ranking signal. The REST route's missing project filter stays #2463's:
   it carries a default-scope UI decision this change must not preempt.

3. The instructions (#119, _INSTRUCTIONS + using-scribe skill; plugin 0.1.25
   for the cache):
   - Tag as you write, with an executable test — "would someone investigating
     that subsystem want this in the pile list_system_records returns?" —
     rather than "tag appropriately", which is what died.
   - Create the System when the area has no record: the two-or-more test
     snippets use, plus "don't wait to be asked to name an area that plainly
     exists", because the agent's default was leaving un-modelled areas
     un-modelled forever.
   - State vs chronicle: dev-logs are written once and never rewritten; durable
     findings live in the System's reference note, updated in place — safe
     because note versions are the changelog, which has existed since the
     feature shipped and was never named as one.

list_system_records' docstring now sells it as the way to READ a subsystem,
reference note first. No auto-inject boost by System — vocabulary and filter
first, measure before adding ranking behaviour (the #2486 lesson).

Refs #278, #2546

src/scribe/mcp/tools/search.py

@@ -20,6 +20,7 @@ async def search(
     content_type: str = "all",
     limit: int = 10,
     project_id: int = 0,
+    system_id: int = 0,
 ) -> dict:
     """Semantic search over the user's existing notes and tasks — Scribe's recall.
 
@@ -39,6 +40,11 @@ async def search(
             enter_project) — otherwise this searches across ALL projects and
             bleeds unrelated work into the result set. 0 = search everything
             (use only when you genuinely want a cross-project sweep).
+        system_id: Narrow to records tagged to one System (a named
+            subsystem/area — enter_project lists them). Use when investigating
+            a specific subsystem: it cuts the candidates to records someone
+            deliberately filed under that area. 0 = no system filter.
+            list_system_records gives the same slice unranked.
 
     Returns:
         {"results": [{"id", "title", "body", "is_task", "tags", "similarity"}],
@@ -55,6 +61,7 @@ async def search(
     raw = await semantic_search_notes(
         uid, q, limit=limit, is_task=is_task,
         project_id=project_id or None,
+        system_id=system_id or None,
         # An explicit search reaches everything the operator may read, including
         # records shared with them one-to-one.
         scope="read",