feat(instructions): fit the delivery fold — 2k server map, floor Systems reflex, write-time systems_hint

Claude Code injects only the first ~2,048 chars of an MCP server's
instructions and silently cuts the rest mid-word (#2562, observed live):
_INSTRUCTIONS was 20,002 chars, so ~90% — including all Systems tagging
guidance — never reached any session. Rearchitect delivery around what
each surface actually delivers:

- _INSTRUCTIONS becomes a 1,997-char purpose-sorted map, with a header
  comment stating the budget and where detail belongs instead.
- Tool docstrings keep the per-tool HOW (audit: nearly all displaced
  topics were already duplicated there); backfill the four gaps —
  enter_project session scoping + project bootstrap, create_rule
  entity-vs-rule test, create_design_system not-a-rulebook,
  create_system two-records test.
- The plugin static context (the delivery floor) gains the
  tag-to-Systems reflex and a surfaces-layering statement; plugin
  0.1.25 -> 0.1.26 so the executing cache refreshes (#2209).
- create_task / create_note / create_snippet return a systems_hint when
  a record is created untagged in a project that has Systems — in-band
  at the exact write it applies to, fail-open like the dedup gate.
- Guards: _INSTRUCTIONS length budget, floor-states-the-reflex, and a
  displaced-topics sweep asserting every cut topic still lives on a
  delivered surface.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

src/scribe/mcp/tools/systems.py

@@ -16,6 +16,30 @@ from scribe.mcp._context import current_user_id
 from scribe.services import systems as systems_svc
 
 
+async def untagged_systems_hint(user_id: int, project_id: int) -> str | None:
+    """Nudge text for a record created untagged in a project that has Systems.
+
+    Not a tool. create_task / create_note / create_snippet attach this to
+    their responses so the tagging question arrives in-band at the exact write
+    it applies to — instruction prose alone demonstrably doesn't fire at write
+    time, while in-band behavior (the duplicate gate) does (#2562).
+    """
+    # Fail-open like the dedup gate: a hint must never break a create.
+    try:
+        systems = await systems_svc.list_systems(user_id, project_id)
+    except Exception:
+        return None
+    if not systems:
+        return None
+    names = ", ".join(f"#{s.id} {s.name}" for s in systems)
+    return (
+        f"Created untagged. This project's Systems: {names}. If this record is "
+        "about one of those areas, tag it (update it with system_ids=[...]); if "
+        "its area is missing, create_system it and tag; if it is about no "
+        "particular area, leave it untagged."
+    )
+
+
 async def create_system(
     project_id: int,
     name: str,
@@ -27,6 +51,13 @@ async def create_system(
     Associate records with it via the `system_ids` arg on create/update_task and
     create/update_note.
 
+    Create one the moment two records would share an area that has no System
+    yet — the same two-or-more test snippets use. Don't wait to be asked to
+    name an area that plainly exists in the code; an unmodelled area means
+    every record about it stays untaggable. Give it a one-paragraph charter,
+    not just a label: the description is what tells a later session whether a
+    record belongs here.
+
     Args:
         project_id: The project this system belongs to (required).
         name: Short label (required).