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

tests/test_mcp_tool_projects.py

@@ -17,6 +17,18 @@ def _bind_user():
     _user_id_ctx.reset(token)
 
 
+@pytest.fixture(autouse=True)
+def _no_systems():
+    """enter_project now surfaces the project's Systems as the tagging
+    vocabulary (#2546). These are tool-layer unit tests with no database, so
+    the lookup is stubbed to the common case — a project with none. The
+    populated shape is asserted in its own test below.
+    """
+    with patch("scribe.mcp.tools.projects.systems_svc.list_systems",
+               AsyncMock(return_value=[])):
+        yield
+
+
 def _fake_project(design_system_id=None, **overrides) -> MagicMock:
     p = MagicMock()
     base = {"id": 1, "title": "P", "description": "", "goal": "",
@@ -184,6 +196,48 @@ async def test_enter_project_composes_full_context():
     # absent. A caller that has to distinguish "no key" from "no system" will
     # eventually get it wrong.
     assert out["design_system"] is None
+    # No Systems -> present-and-empty, NOT absent: this key is the tagging
+    # vocabulary, and "this project has no named areas yet" is information the
+    # create-the-System instruction acts on.
+    assert out["systems"] == []
+
+
+@pytest.mark.asyncio
+async def test_enter_project_surfaces_the_systems_vocabulary():
+    """The tagging instruction is only executable if the vocabulary is in
+    front of the agent when it writes. It never was, and tagging stopped three
+    days after the feature landed — one System, nothing tagged since July 28
+    (#2546's audit). Trimmed to id/name/first-line: it rides on every session
+    start, and the full charter is get_system's job."""
+    p = _fake_project(id=5)
+    sys1 = MagicMock()
+    sys1.id = 3
+    sys1.name = "retrieval"
+    sys1.description = "Embeddings, ranking, auto-inject.\nLong detail below."
+
+    with patch(
+        "scribe.mcp.tools.projects.projects_svc.get_project",
+        AsyncMock(return_value=p),
+    ), patch(
+        "scribe.mcp.tools.projects.rulebooks_svc.get_applicable_rules",
+        AsyncMock(return_value={"rules": [], "truncated": False,
+                                "subscribed_rulebooks": []}),
+    ), patch(
+        "scribe.mcp.tools.projects.milestones_svc.get_project_milestone_summary",
+        AsyncMock(return_value=[]),
+    ), patch(
+        "scribe.mcp.tools.projects.notes_svc.list_notes",
+        AsyncMock(side_effect=[([], 0), ([], 0)]),
+    ), patch(
+        "scribe.mcp.tools.projects.systems_svc.list_systems",
+        AsyncMock(return_value=[sys1]),
+    ):
+        out = await enter_project(project_id=5)
+
+    assert out["systems"] == [
+        {"id": 3, "name": "retrieval",
+         "description": "Embeddings, ranking, auto-inject."}
+    ]
 
 
 @pytest.mark.asyncio