CL-9026: stop writing duplicate prompt snapshot when identical to live turns

[TheGreatAxios]

Summary

The reactor checkpoints the materialized prompt every cycle, but most cycles run no transform that changes it. The optimized context store wrote that identical snapshot to prompt.jsonl unconditionally, doubling disk and re-hash cost for zero information. writePrompt now hash-compares (per-turn JSON.stringify, the writer's own serialization) against live turns and skips the write entirely when identical.

Behavior

• Identical prompt (by reference fast path, then serialized equality) against unpublishedRewrite ?? liveTurnRefs in memory, else on-disk live turns for fresh instances: writer is never invoked, no prompt segment is created.
• Identical-after-differing converges disk to no-prompt-segment: stale prompt segments are unlinked, their removals staged so commit drops them from the tree, and the prompt writer is reset. Nothing reads prompt.jsonl (load merges base turns + turns extras only), so resume falls back to live turns by omission.
• Differing prompts (compaction folds, cache-shrinks, ephemeral appends) write exactly as before. Any baseline-read failure fails open to a normal write.
• Old sessions: byte-identical snapshots become prompt no-ops on the first post-upgrade checkpoint; pre-existing differing prompts are preserved until replaced/removed.

Tests

• T1 identical writeTurns then writePrompt: prompt.jsonl absent on disk and in commit tree, including a fresh-instance writePrompt against on-disk turns.
• T2 differing (shrunk tool-result body, plus ephemeral trailing turn): prompt.jsonl present with exact content.
• T3 turns-only store: load returns live turns; fresh-instance commit is a hash-identical no-op that invents no prompt file.
• T4 differing-then-identical: stale segments removed from disk, removal staged, tree clean after commit, load unchanged.

Fixes CL-9026

[linear-code]

CL-9026

[github-actions]

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