fix(coding-agent): preserve custom message timestamps

[rynfar]

Custom messages received a new timestamp when saved, so rebuilding the same session changed its transcript identity. Preserve the original message timestamp at all eight AgentSession persistence sites and reject invalid explicit timestamps before mutating history.

Fixes #76. This is required by Pylon PrimeIntellect-ai#552 and the managed delivery tracked in pylon-code/pylon#557. Existing histories whose timestamps already differ remain unproved; this change does not rewrite them or weaken continuity checks. No daemon protocol or artifact recipe changes.

Validation: 165 focused regression/session tests passed; npm run check and the required commit hook passed. Regression coverage advances the persistence clock and compares live, rebuilt, compacted, and reopened history, plus rollback and invalid timestamps.

Maintainer authorization: the developer requested review, fixes, and merging this batch (“finish these please”). Merge after green hosted checks.

Implemented and reviewed with GPT-6 Astra in Pylon/Codex.

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