Fix hook crashing on every call under the host's Python 3.9

[agb1986]

Summary

Diagnosed via a handoff from a prior debugging session on the CasaOS host: no data was reaching Mongo/:8743 despite the hook being installed and finance-agent producing artifacts. Two independent bugs, this PR fixes the first.

• log_artifact_entry.py uses PEP 604 union syntax (str | None, dict | None) in three signatures (render, find_project_root, build_entry). This requires Python 3.10+ to evaluate at def time. The Docker image is unaffected (python:3.12-slim), but the hook runs directly on the host via ~/.claude/settings.json's PostToolUse (python3 ~/.claude/hooks/log_artifact_entry.py) — and the host's only python3 is 3.9.2. Result: TypeError on every single invocation, in every project, since install — silent, since hook stderr isn't surfaced anywhere visible.
• Fix: from __future__ import annotations as the first import, deferring annotation evaluation to strings. No syntax rewrite, no host Python upgrade needed. Confirmed no other 3.10+ constructs exist in the file (only those three annotations use |; no match statements; all isinstance calls use plain builtin types).

A second, independent bug from the same handoff — finance-agent never had a .ai-data-store.json to opt in — is fixed separately in agb1986/finance-agent#13, since that config lives in finance-agent's own repo.

Test plan

• ruff check . — clean
• pytest — 57 passed
• Confirmed by grep that the three flagged lines are the only PEP 604 usages in the file, and that no other constructs in it require Python ≥ 3.10 at runtime
• Requires the CasaOS host to apply this — I have no access to that host from this session (this ran on the dev machine, not casaos). After merge, on the host:

  git pull
  cp hooks/log_artifact_entry.py ~/.claude/hooks/log_artifact_entry.py

  Then, once Opt in to the ai-data-store artifact-logging hook finance-agent#13 is also merged and pulled there, re-run the handoff's smoke test (artifact JSON → hook invocation → Mongo count increments → visible on :8743), and clean up the test doc/tmp file afterward.

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