refactor: centralize title hydration

[JoernZheng]

Problem

Session title hydration is implemented three times: loading a history slot,
restoring recent sessions, and resuming a slot through the dashboard API. Each
copy must independently preserve display redaction, title provenance, and the
refresh milestone.

Why it matters

These copies currently agree, but a future title-metadata change can easily miss
one resume path. That would make the same persisted session behave differently
depending on how it was restored, including silently losing title-refresh state.

Fix (symptoms → root cause → change)

The repeated blocks all implement one hydration contract, so this change moves
that contract into _rehydrate_slot_title and uses it from all three callers.
The helper keeps the existing URL and credential redaction order, _titled
state, legacy title-origin fallback, and refresh-mark normalization unchanged.

Tests

• Added a focused contract test covering redaction order, title state,
  provenance, and refresh-mark restoration.
• python -m pytest -q test/test_title_refresh.py (64 passed).
• Changed-file isort, flake8, and mypy checks passed.
• npm run build passed.
• The repository-wide backend suite reached 39,896 passed; its 28 failures are
  confined to untouched host/integration tests (local resource injection, Xcode
  trust, worktree nesting, and host tool paths).

Manual verification

N/A — the change is a behavior-preserving backend refactor and the three hydration
paths are covered by automated tests.

Closes #2288

[github-actions]

Opus 4.8 Review (fork) — ✅ no blocking findings

Reviewed 53de71ccebf013c39cae7724437b66d6ae780e9e via the fork AI-review pipeline; updated in place on each push.

Review details

No findings.

This is a behavior-preserving refactor: the three call sites now delegate to _rehydrate_slot_title, which reproduces the original redaction order (redact_exfiltration_urls → redact_credentials), title assignment, and _titled/_title_origin/_title_refresh_mark restoration exactly — including using the titled value for _title_origin. Redaction of LLM-influenced titles is preserved on every path.

[OPUS-REVIEWED] 53de71c

[github-actions]

GPT 5.6 Review (fork) — ✅ no blocking findings

Reviewed 53de71ccebf013c39cae7724437b66d6ae780e9e via the fork AI-review pipeline; updated in place on each push.

Review details

No findings.
[GPT-REVIEWED] 53de71c

[github-actions]

Design Review (Fable 5, fork) — ✅ PASS

Advisory design-level review of 53de71ccebf013c39cae7724437b66d6ae780e9e via the fork AI-review pipeline — updated in place on each push; does not block merge.

Design-Verdict: PASS

Faithful extraction of a genuinely triplicated hydration contract into the module that already owns slot persistence; all three call sites map 1:1 with no semantic drift.

[DESIGN-REVIEWED] 53de71c