feat(store): expose eligible pending mutation high-water

[dnlrsls]

🔗 Linked Issue

Closes #1458

Child of tracker #1464. Target feat/prompt-inbox-foundation-tracker, not main; no remote delete behavior is enabled by this helper alone.

🏷️ PR Type

• type:bug — Bug fix
• type:feature — New feature
• type:question — Question requiring tracked work
• type:docs — Documentation only
• type:refactor — Code refactoring
• type:chore — Maintenance
• type:breaking-change — Breaking change

📝 Summary

• Add a read-only, target-scoped maximum eligible pending mutation sequence for a future bounded autosync pass.
• Test that stale sync_state.last_enqueued_seq does not hide pending eligible rows; exclude other targets, unenrolled, acknowledged and quarantined rows.

📂 Changes

File	Change
internal/store/store.go	Query max seq using the same target/enrollment/pending predicates as the existing pager; return zero for no eligible row.
internal/store/store_test.go	Direct-SQL eligibility and stale-state regression.

71 additions, no transient artifacts. Documentation N/A: the new store read is not wired to autosync until a separate client preflight PR.

🧪 Test Plan

• Focused regression: go test ./internal/store -run '^TestMaxPendingSyncMutationSeq$' -count=1 — RED compile before implementation, GREEN afterward; independent PASS.
• Affected package: go test ./internal/store -count=1 — independent PASS.
• Other: go vet ./internal/store, git diff --check, CRLF-normalized gofmt comparison — independent PASS. Native review review-7c24726375ab72ae approved/acknowledged.
• GitHub full unit/E2E/plugin/lint/Windows and policy checks — pending until run.

🤖 Automated Checks

All pending after opening; do not infer CI from local tests. Required checks must pass before normal merge.

✅ Contributor Checklist

• Approved issue feat(store): preserve admitted prompt identity through sync and deletion #1458 linked above.
• Exactly one type:* label (type:feature).
• Focused/package/additional local checks recorded; CI pending.
• Docs N/A until user-visible behavior is enabled by follow-up.
• Conventional commit, no co-author trailer.
• Changed paths comply with transient-artifact policy.

💬 Notes for Reviewers

Compare this query with ListPendingSyncMutationsAfterSeq: the predicates must agree. It is a prerequisite for a bounded autosync traversal that can skip an unverified full first page without starving later healthy rows. T4b3a2b will integrate it; T4b3b explicitly reauthorizes old/imported sources via per-source CLI; T4c still gates deletes remotely. Tracker #1464 stays outside main queue.

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