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/snippets.py

@@ -11,6 +11,7 @@ on create, System association passthrough).
 from __future__ import annotations
 
 from scribe.mcp._context import current_user_id
+from scribe.mcp.tools import systems as systems_tools
 from scribe.services import access as access_svc
 from scribe.services import dedup as dedup_svc
 from scribe.services import snippets as snippets_svc
@@ -132,7 +133,9 @@ async def create_snippet(
     and it really is the same reusable thing found in another place, prefer
     merge_snippets(existing_id, [new...]) — or record then merge — to unify them
     into ONE canonical record (which then carries every call site as a location),
-    rather than forcing a second copy with force=true.
+    rather than forcing a second copy with force=true. Created untagged in a
+    project that has Systems, the response carries a `systems_hint` naming
+    them — answer it: tag, create the missing System, or deliberately skip.
 
     WHAT THE GATE MATCHES ON. Exact identity first — an existing snippet at the
     same repo · path · symbol, or holding byte-identical code. Those are certain,
@@ -175,6 +178,10 @@ async def create_snippet(
         data["systems"] = [
             s.to_dict() for s in await systems_svc.list_record_systems(uid, note.id)
         ]
+    elif project_id:
+        hint = await systems_tools.untagged_systems_hint(uid, project_id)
+        if hint:
+            data["systems_hint"] = hint
     return data