feat(inbox): C2.5 — inbox hardening + SDK version guard (closes C-F7, C-N3, R-6)

- File-layer decision vocabulary (§4.2 set) with SKIP semantics — an unknown
  decision never reaches the store (C-F7, the review's run proof is the fixture)
- Fail-fast caps (max_files / max_rationale_chars) via InboxLimitError raised
  OUTSIDE the tolerant try — a cap breach is never swallowed as a skip
- R-6 id grammar (mirrors ingest _ID_RE) as a pydantic pattern on
  VerdictDocument.id AND re-checked in write_verdict, since model_copy(update=)
  bypasses model validation — traversal ids can no longer write outside the inbox
- promotion._filename_token: any sanitised id maps to a content hash — 'e/vil'
  can no longer clobber the distinct id 'evil' (restarbeid-funn 2)
- SDK pinned >=0.2.111,<0.3 + version guard test naming the sdk_client.py
  attribute premises; resolved 0.2.120, all premises re-verified against it
- sdk_client read loop bound offline with REAL SDK message types (R-4/R-5):
  text aggregation, error fail-paths, usage/cost extraction, _total_tokens
  fail-closed, non-positive budget guard
- test_sdk_isolation comment no longer claims the --system-prompt ""
  serialization the test body does not bind (honesty rule §1)

Guard-G2 assessment (guard-plan §4): the allowlist + caps + id grammar landed
here are G2's necessary part; an optional scan_output depth pass over
rationale (still a verbatim prose channel into the fold prompt, R-9) remains
relevant as a later additive session — the trigger picture is unchanged.

4 detach proofs red → restored green. Full gate: 389 passed (365→389),
ruff+format+mypy clean; golden + shared/ + runs/s10/ byte-untouched.

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

pyproject.toml

@@ -5,7 +5,7 @@ description = "Sibling implementation of the portfolio-optimiser method on the C
 readme = "README.md"
 requires-python = ">=3.10"
 dependencies = [
-    "claude-agent-sdk>=0.2",
+    "claude-agent-sdk>=0.2.111,<0.3",
     "pydantic>=2",
 ]