fix(agentdb): Add string ID mapping to RuVectorBackend to prevent silent data loss

The @ruvector/core N-API layer converts IDs via Number(), causing non-numeric
string IDs (UUIDs, hex hashes, prefixed IDs) to become NaN and be silently
dropped. Added bidirectional idToLabel/labelToId mapping in the AgentDB layer
as defense-in-depth alongside the upstream fix in @ruvector/rvf@0.2.0.

- insert(): Maps string IDs to sequential numeric labels before N-API call
- search(): Maps numeric labels back to original string IDs in results
- remove(): Resolves string ID to numeric label before N-API call
- save()/load(): Persists ID mappings in .meta.json sidecar (backward-compatible)
- Updated ruvector dependency to ^0.1.99
- Bumped version to 2.0.0-alpha.2.12

Closes #114

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: ruvnet