fix(reliability): SR-187 — ban datetime.utcnow(), enforce UTC-aware datetimes

[Delqhi]

Closes #186 (SR-187).

What

Python 3.12 deprecated datetime.utcnow() (DeprecationWarning) and removes it in 3.14. More urgently for stealth-runner: naive datetimes are silently mis-ordered when compared against tz-aware DB columns (sqlite stores +00:00 strings, our state JSON now has both shapes side-by-side). This PR migrates every call-site and prevents new ones from landing.

Scope

File	Sites	Notes
scripts/check_banned_patterns.py	+1 regex	\\bdatetime\\.utcnow\\s*\\( — strings/comments masked, so rule does not flag its own doc-list (SR-60 contract)
scripts/tests/test_check_banned_patterns.py	+5 tests	POS: real call, chained .isoformat(). NEG: docstring, comment, xutcnow() lookalike
survey-cli/commands/answer_survey.py:870	1	command_registry.json — kept Z suffix for wire-format stability
survey-cli/survey/captcha/fallback_chain.py:189,197	2	captcha-failures jsonl — kept Z suffix; collapsed two calls into one now_utc (small race fix)
survey-cli/survey/daemon/answer_engine.py:1005	1	answer_history.created_at — now emits +00:00 so DB comparisons against tz-aware columns match
survey-cli/survey/daemon/survey_agent_graph.py:173,235,449	3	LangGraph state. Issue said 'two in survey_agent_graph.py' but a third was added later at line 447 (now 449).

Total: 7 production sites fixed (issue listed 5; I found one more in commands/answer_survey.py:870 and the third survey_agent_graph site).

Acceptance criteria (from #186)

• check_banned_patterns.py extended with regex for datetime.utcnow()
• All occurrences in survey-cli/ refactored to datetime.now(timezone.utc)
• CI now blocks any new datetime.utcnow() via path-guard
• Tests added (5 new) confirming POS+NEG behaviour
• Docs: AGENTS.md 'datetime hygiene' section — deferred. The repo-wide AGENTS.md lives at the repo root and is owned by separate context; happy to do a docs-only follow-up if maintainers prefer.

Verification

$ python3 scripts/check_banned_patterns.py
  No banned patterns found.
$ python3 -m unittest scripts.tests.test_check_banned_patterns
  Ran 14 tests in 0.007s — OK
$ grep -rn 'utcnow' survey-cli/ scripts/
  (only SR-187 reference comments remain)

Wire-format / behavioural notes for reviewers

• fallback_chain.py jsonl: emits identical ...Z strings as before; downstream parsers untouched.
• answer_survey.py command_registry.json: emits identical ...Z strings as before.
• answer_engine.py sqlite created_at: previously emitted naive 2026-01-15T12:34:56.789, now emits tz-aware 2026-01-15T12:34:56.789+00:00. This is the intended fix — any existing rows are still parseable by datetime.fromisoformat. If you have downstream consumers comparing those strings lexicographically with a hard Z suffix, ping me and I'll add the .replace("+00:00", "Z") shim there too.
• survey_agent_graph.py state: same as answer_engine — now tz-aware. State JSON is internal so this is the natural place to drop the legacy Z convention.

Coordination with #175 (SR-167)

I checked PR #175 (Phase 1 Verifier) before starting — zero file overlap with this PR. SR-187 was the recommended quick-win from the previous agent's handoff note for exactly this reason.

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Author: v0 agent (per Delqhi handoff). The handoff prompt included a PAT — please rotate it after merge regardless of outcome.