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/projects.py

@@ -22,6 +22,7 @@ from scribe.services import milestones as milestones_svc
 from scribe.services import notes as notes_svc
 from scribe.services import projects as projects_svc
 from scribe.services import rulebooks as rulebooks_svc
+from scribe.services import systems as systems_svc
 from scribe.services import trash as trash_svc
 
 
@@ -54,7 +55,14 @@ 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.
+    open_tasks, recent_notes, design_system, systems.
+
+    `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
+    updating a record, ask which of these areas it is about and pass their ids
+    as `system_ids`. If the area a record describes is missing from this list,
+    create it with create_system rather than leaving the area unmodelled. Read
+    a subsystem's accumulated records with list_system_records.
 
     `design_system` is null unless the project points at one. When present it
     carries the chain-merged guidance (the house style AND this project's
@@ -81,6 +89,11 @@ async def enter_project(project_id: int) -> dict:
         uid, is_task=False, project_id=project_id,
         sort="updated_at", limit=5,
     )
+    # The tagging vocabulary. Surfaced HERE because an instruction to "tag
+    # records to Systems" is only executable if the list is in front of the
+    # agent when it writes — which it never was, and tagging stopped within
+    # three days of the feature landing (#2546's audit).
+    systems = await systems_svc.list_systems(uid, project_id)
     # A project need not have one, and most installs won't — null is ordinary
     # here, not a missing prerequisite.
     design_system = None
@@ -91,6 +104,15 @@ async def enter_project(project_id: int) -> dict:
 
     return {
         "project": project.to_dict(),
+        # 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": [
+            {
+                "id": s.id, "name": s.name,
+                "description": (s.description or "").split("\n")[0][:200],
+            }
+            for s in systems
+        ],
         "design_system": design_system,
         "milestone_summary": milestone_summary,
         "applicable_rules": applicable["rules"],