feat(rules): retrieval honours a rule's home — global everywhere, a project's rules only in that project (#4074)

semantic_search_rules searched every rule the user owned, and every hook arm
called it without a project, so each project's rules were injected into every
other project's sessions and a project rule meant nothing a session could feel.

The search now takes a scope: global rules by default (an unbound session, or a
caller that forgets to say), global plus project N when given project_id (N's
rules only if the caller can read that project, through access.can_read_project),
and every owned rule with everywhere=True. The four hook arms and the report
preference lookup pass the session's project; an explicit
search(content_type="rule") scopes to its project_id, or asks the whole rulebook
without one.

Milestone 414 step 1. Guarded by an AST walk that every hook call site passes
project_id, and an integration test on real Postgres that a rule is reached only
from its home.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01821k5B3Ysecp9fNYs92Kuy

src/scribe/services/reply_preferences.py

@@ -107,7 +107,7 @@ async def completion_preferences(user_id: int, *, project_id: int | None = None)
         t0 = time.perf_counter()
         hits = await semantic_search_rules(
             user_id, COMPLETION_QUERY, limit=LIMIT, threshold=threshold,
-            kind="preference", report=report,
+            kind="preference", report=report, project_id=project_id,
         )
         hits = [(score, rule) for score, rule in hits if rule.kind == "preference"]
         record_retrieval(