fix(store): refactor Import() with LWW conflict resolution

[Snakeblack]

Summary / Resumen

Replaces the blind INSERT-only loops in Store.Import() with sync_id-keyed conflict resolution. Observations now resolve via Last-Write-Wins by updated_at; prompts use skip-if-exists (they are immutable). deleted_at is treated as part of the LWW state so soft-deletes propagate across machines and active rows can be restored when an importer's payload is newer.

Changes

Store Layer

• Import() rewritten:
  • Observations: SELECT id, updated_at WHERE sync_id = ? → INSERT / UPDATE (full row incl. deleted_at) / SKIP based on payload.UpdatedAt > local.UpdatedAt.
  • Prompts: SELECT id WHERE sync_id = ? → INSERT if absent, SKIP otherwise.
• ImportResult expanded to granular counters: SessionsImported, ObservationsInserted/Updated/Skipped, PromptsInserted/Skipped.
• Backfill SQL (lines 873, 884, 5230) now uses randomblob(8) so every newly generated sync_id matches newSyncID's 16-hex-char format. Existing IDs in either format are preserved by normalizeExistingSyncID.

Sync Layer

• internal/sync.ImportResult mirrors the new granular fields and aggregates them across chunks.
• estimateMutationImportResult feeds the new Inserted counters (best-effort estimate from mutation entities; LWW happens inside the store).

CLI

• engram import and engram sync print the new counters.

Tests (9 new)

• TestImportLWWIdempotentReimport — re-importing the same dump produces zero inserts and N skips.
• TestImportLWWUpdatesNewerObservation — payload with newer updated_at updates the local row.
• TestImportLWWSkipsOlderObservation — payload older than local is skipped, local content preserved.
• TestImportLWWPromptsAreImmutable — re-importing a prompt with edited content does not overwrite local.
• TestImportLWWPreservesDeletedAtOnInsert — soft-deleted obs imported into empty store keeps deleted_at.
• TestImportLWWSoftDeletePropagatesViaLWW — payload soft-delete with newer updated_at propagates locally.
• TestImportLWWRestorationViaLWW — payload reactivation with newer updated_at clears deleted_at.
• TestImportResultHasGranularCounters — asserts the new struct shape.
• TestNewSyncIDFormatConsistency — newSyncID(prefix) always returns prefix-{16 hex chars}.

Existing tests using the old ObservationsImported / PromptsImported fields were updated to read the new *Inserted counters.

Migration impact

• No DB schema migration. No UNIQUE(sync_id) constraint added.
• Existing sync_id values (16-hex from Go, 32-hex from old backfill) are accepted as-is; normalizeExistingSyncID keeps coexistence working.
• Backfill format change only applies to NEW rows that go through the migration with a NULL/empty sync_id — already-backfilled rows are untouched.
• Public API change: callers reading ImportResult.ObservationsImported / PromptsImported must update to the new field names. Updated in this PR for cmd/engram and internal/sync.

Closes #244