fix: retain exact reads through agent compaction

[jalehman]

What

Preserve exact successful read results through bounded provider-context compaction, and require source-bound ingestion skills to stage each update immediately after its sole exact baseline read.

Why

Production Terra canary job 2796 batched several large get_page calls. The existing uniform per-round projection removed every exact page body and content hash from provider-visible context, so the agent could not freeze a CAS-guarded proposal even though the durable tool ledger remained complete. The job failed safely with no writes, but it could not make progress.

Changes

• Optimize exact read retention within byte caps
• Derive mutation identity from assistant calls
• Project mismatched tool-result identities safely
• Keep durable tool history immutable
• Serialize baseline reads and proposal staging
• Add focused compaction and skill regressions

Testing

• bun test test/ai/tool-loop-context-exact-results.test.ts test/granola-meeting-ingestion-skill.test.ts test/github-project-ingestion-skill.test.ts test/gmail-thread-ingestion-skill.test.ts — 59 pass
• bun run verify — 31/31 pass
• TypeScript — clean
• Autoreview — clean
• Independent thermonuclear review — clean

[jalehman]

Exact-head CI assessment for f0498405fcd0afa7cb34a564e8890442a9f50612:

• Passed: Tier 1, Tier 2, JSONB parity, verify, serial tests, cache, gitleaks, semgrep, and both slow focused jobs.
• Local changed-path proof: 59 focused tests passed; repository verify 31/31 passed; TypeScript, autoreview, and independent strict review clean.
• Broad shards 4, 5, 7, 9, and 10 failed. Every concrete failure reproduces in the deployed-base push run 31238867746 for 5b909037: stale handler expectations, existing PGLite/extract/source-state failures, stale global skill-conformance headings, and replace-page-text-operation.test.ts:138. Shard 4 later hit its 15-minute timeout after reporting that same base failure.
• None of the failed tests or stack locations touch the compactor, its focused test, or the three changed ingestion skills.

The hosted suite remains red at its inherited baseline; the exact changed contracts and meaningful gates are green.