fix(runtime): repair and stamp unfenced journal frontmatter blocks

[dembrane-sam-bot]

What is this change?

This change introduces a runtime repair function _ensure_fenced_frontmatter in src/runtime/adk_runner.py that detects unfenced or unterminated frontmatter blocks in journal entries (e.g., blocks starting straight with trigger: instead of --- ). It wraps them in proper YAML fences so that _stamp_journal_frontmatter can correctly stamp the runtime-derived date: and session: fields, preventing un-greppable and malformed entries from landing in the journal. It also includes comprehensive tests in tests/runtime/test_journal_append.py.

What did Sam notice that led to this?

During the daily maintenance on 2026-08-20, an audit of the previous day's (2026-08-19) journal revealed roughly 12 entries that lacked date: and session: fields entirely. While the errored sessions were properly stamped, successful sessions that skipped the opening --- fence were written directly without date/session stamps. The Claude Opus mentor confirmed that the runtime only stamped frontmatter if the model-authored entry started with ---\n, creating a silent data loss for successful but slightly misformatted entries.

Tier

• Tier 3 (substrate/runtime behavior change). Requires principal operator review to merge.

Confidence

• Fully confident. The implementation preserves the prose-only (non-frontmatter) continuation logic, correctly normalizes unfenced/unterminated blocks, and passes all 658 suite tests.

[spashii]

Reviewed and approved by Sameer. Rebased onto main; the prior CI failure was a stale .trivyignore.