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/services/embeddings.py

@@ -209,6 +209,7 @@ async def semantic_search_notes(
     orphan_only: bool = False,
     scope: str = "own",
     demote_superseded: bool = True,
+    system_id: int | None = None,
 ) -> list[tuple[float, Note]]:
     """Return up to *limit* (score, note) pairs most relevant to *query*.
 
@@ -281,6 +282,19 @@ async def semantic_search_notes(
                 stmt = stmt.where(Note.project_id.is_(None))
             elif project_id is not None:
                 stmt = stmt.where(Note.project_id == project_id)
+            # Narrow to records tagged to one System (subsystem/area). An
+            # association filter, not a ranking signal — membership in the
+            # candidate set, decided before scoring, like project_id above.
+            if system_id is not None:
+                from scribe.models.system import RecordSystem
+                stmt = stmt.where(
+                    select(RecordSystem.id)
+                    .where(
+                        RecordSystem.note_id == Note.id,
+                        RecordSystem.system_id == system_id,
+                    )
+                    .exists()
+                )
             if is_task is True:
                 stmt = stmt.where(Note.status.isnot(None))
             elif is_task is False: