[WRONG BRANCH] fix(cursor): bind ref-less prefix checkpoints to conversation id

[luvs01]

Motivation

• Prevent ref-less checkpoint reuse from importing prior ConversationStateStructure into an unrelated chat when a unique message prefix exists, which could leak or influence model-visible prior assistant state across conversations.

Description

• Require the checkpoint's recorded conversationId to match the active request before accepting a ref-less prefix match by adding a conversationId parameter to getCursorCheckpointForPrefix and checking it during lookup (src/adapters/cursor/checkpoint-store.ts).
• Pass the active request.conversationId into the prefix lookup path and keep the conversation-lineage gate after resolution (src/adapters/cursor/request-builder.ts).
• Tighten the conversation-change invalidation so ref-less and explicit-ref paths both enforce conversation affinity where appropriate (src/adapters/cursor/request-builder.ts).
• Update tests to reflect isolation semantics (isolated helper turns must not inherit parent checkpoints) and add a regression that proves a unique covered-prefix cannot be reused across distinct Cursor conversations (tests/cursor-request-builder.test.ts).

Testing

• Ran focused unit tests: bun test tests/cursor-request-builder.test.ts — all tests passed (44 passed, 0 failed).
• Ran static checks: bun run typecheck — success; bun run privacy:scan — success.
• Ran the full repository test suite: bun run test — the changed Cursor-focused tests passed within the full run, but the complete suite reported unrelated failures in other subsystems (environment/service/timing-sensitive tests); these failures are outside the Cursor changes and were not caused by this fix.

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[github-actions]

✅ Deterministic PR hygiene checks passed.

[github-actions]

⏳ DRAFT

• wrong target branch (main); retarget to dev.

What to do

• Retarget this PR to dev — all contributions go to dev.

Its title has been prefixed with [WRONG BRANCH].
This pull request was already a draft. Its draft status will be preserved after every issue above is resolved.