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/routes/search.py

@@ -34,10 +34,15 @@ async def search_route():
     content_type = request.args.get("content_type", "all")
     limit = min(request.args.get("limit", 10, type=int), 50)
     is_task = _content_type_to_is_task(content_type)
+    # Same association filter the MCP tool takes (#33). The project filter this
+    # route is still missing is #2463's — it carries a default-scope UI decision
+    # this change must not preempt.
+    system_id = request.args.get("system_id", type=int)
 
     t0 = time.perf_counter()
     results = await semantic_search_notes(
         uid, q, limit=limit, is_task=is_task, threshold=_REST_SEARCH_THRESHOLD,
+        system_id=system_id,
         # The user typed this, so it reaches everything they may read.
         scope="read",
     )