refactor(skills): using-scribe keeps the every-turn practices; moment-specific depth moves to reference files (#4398)

Anthropic's skill guidance: keep SKILL.md under 500 lines, split into
reference files linked one level deep as it nears that. using-scribe was
478 and every new practice lands there.

- SKILL.md 478 -> 317 lines. It keeps orientation, one copy, the reflexes,
  scope, the judge section, UI and the process-skill index, plus a "Read
  these when the moment comes" list naming each file with its moment.
- projects.md: binding a non-git directory (.scribe) and project inception.
- writing-records.md: where a new rule goes, lesson growth, and notes that
  carry their own check (reflex 10 keeps a pointer).
- missed-retrieval.md: the record-before-dial route, verbatim.
- Text moved, not rewritten, except for the seams and one cross-reference.

Tests:
- tests.helpers.skill_text reads SKILL.md plus its reference files. The
  ownership registry, the miss-route and the verification tests use it, so
  a topic stays owned by its skill whichever file holds it.
- The force test scans every skill .md on its own, since each file is read
  on its own.
- New test_skill_structure: SKILL.md <= 350 lines, every reference file is
  linked from SKILL.md, none links another, and one over 100 lines opens
  with Contents. Each guard is shown to fail.

The plugin version is minted. That also clears 4fb53b8's red Plugin hooks
lane, which failed only because PACKAGING.md changed without a mint.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tests/test_instruction_surfaces_agree.py

@@ -45,11 +45,12 @@ def _all_surfaces() -> list[tuple[str, str]]:
     the push channel to the operator installing the plugin, and telling a human
     what the hook does is not the same act as telling an agent it need not pull.
     The boundary is "does a session read this", which is skills (loaded by
-    description match), the hook-injected static context, and the MCP server's
+    description match, and each reference file a skill links is read on its
+    own, so it is its own surface — #4398), the hook-injected static context, and the MCP server's
     own instructions.
     """
     found = [(str(p.relative_to(ROOT)), p.read_text())
-             for p in (ROOT / "plugin" / "skills").rglob("SKILL.md")]
+             for p in (ROOT / "plugin" / "skills").rglob("*.md")]
     found += [(str(p.relative_to(ROOT)), p.read_text())
               for p in (ROOT / "plugin" / "hooks").glob("*.md")]
     server = ROOT / "src" / "scribe" / "mcp" / "server.py"