fix(coding-agent): recover deterministic compaction fallback in reasoning sessions

[realsigridjin]

What

Fixes a permanent required-compaction wedge in reasoning-model sessions. When summarization failed, the deterministic fallback rejected provider signature fields, lacked autonomous-turn cut points, and overcounted serialized bytes as tokens, so it returned undefined forever.

Fix

• Accept opaque string thinkingSignature, textSignature, and thoughtSignature fields while keeping malformed values fail-closed.
• Require a non-empty thinkingSignature for redacted thinking blocks.
• Add a deduplicated fallback ladder: prepared, latest user, latest custom/bash turn start, latest assistant cut point.
• Replace bytes-as-tokens with a conservative ceil(bytes / 3) sizing floor.
• Add regression coverage and update changes.md.

Verification

• Focused fallback suite: 18/18 passed.
• Full compaction and adjacent suites: 51 files, 403 tests passed.
• Root npm run check: exit 0.
• Real wedged-session replay: fallbackDefined=false before; after this fix fallbackDefined=true, retained suffix prepared, estimated tokens 120085 under budget 255616.
• Independent code review: APPROVE / CLEAR.
• Hands-on QA: PASSED.

Commits: bb08d0e95, 343c0c9de.

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Summary by cubic

Recovered deterministic compaction fallback for reasoning sessions and replaced the per-turn compaction cap with an absolute session cap. This prevents required-compaction wedges and lets autonomous stretches recover cleanly.

• Bug Fixes
  • Accept opaque string replay signatures on assistant text, thinking, and tool-call blocks; require a non-empty thinkingSignature when redacted: true.
  • Add a deduplicated fallback ladder: prepared → latest meaningful user → latest non-user turn start (custom or bashExecution) → latest assistant cut point.
  • Use a conservative token floor: ceil(UTF-8 bytes / 3) instead of bytes-as-tokens.
  • Replace the per-turn soft cap with an absolute hard cap (hardCap=10); update rejection messages and logs.
  • Document fallback recovery and sizing changes in CHANGELOG.md and builtin compaction changes.md.

^{Written for commit ba1ec5c. Summary will update on new commits.}