feat(store): preserve admitted prompt identity through sync and deletion

[dnlrsls]

🔍 Problem Description

OpenCode V2 emits durable session.inbox.enqueued events that can replay after restart. PR #1240 deduplicates a live local prompt by (session_id, source_inbox_id), but sync/export/import drops that identity and deletion retains only a sync_id tombstone. After restore or delete, a replay can insert the same admitted message again. This cross-package storage and HTTP contract exceeds adapter issue #1220 and needs its own approval.

💡 Proposed Solution

Introduce optional source_inbox_id for prompt writes through the local store and /prompts. Keep a unique identity per (session_id, source_inbox_id) for nonempty IDs: replay returns the existing result without a new row, sync mutation, or write notification; distinct IDs with identical text remain distinct. Carry that identity in prompt sync push/pull and export/import, including restored rows. On deletion retain identity in the existing tombstone lifecycle; replays of the deleted inbox ID must not resurrect a prompt or enqueue an upsert, including after sync/restore. A new inbox ID is a new message. Existing clients omitting the field keep append-per-request behavior. Preserve project ownership and previous wire compatibility.

Acceptance: focused store and server regressions for same-key replay, same text/different IDs, legacy no-ID clients, sync push/pull plus export/import round-trip, and delete-then-replay locally and after restore; assert no redundant notification or sync mutation. Document the optional HTTP field and deletion semantics. No V2 hook, installer, UI, or generic cross-version adapter framework in this issue.

📦 Affected Area

Store (database, queries), local HTTP API, and Sync (multi-instance).

🔄 Alternatives Considered

AddPromptIfMissing uses content as identity and collapses distinct admitted messages with identical text; an in-memory adapter set is lost on restart. Allowing the deleted key to reinsert would undo an intentional deletion.

📎 Additional Context

Foundation dependency for #1240; approved #1220 remains the V2 adapter/setup feature. Build this PR from main so the adapter diff is not imported. Preserve contributor history: after the foundation merges, integrate it into #1240 with a normal merge, not a rebase or force-push.