Fix Postgres embedding cache scoping

[3em0]

Summary

• scope Postgres embedding cache lookups by project_id, content_hash, and an embedder cache namespace
• persist embedding_cache_namespace alongside chunks and index it with the content hash
• derive the namespace from embedder provider/model/dimensions/endpoint and pass it from CLI/MCP store construction
• add regression coverage for SQL scoping, namespace derivation, and optional Postgres integration behavior

Fixes #251

Testing

• docker run --rm -v "$PWD:/src" -w /src golang:1.24-bookworm go test ./config
• docker run --rm -v "$PWD:/src" -w /src golang:1.24-bookworm go test ./store -run TestLookupByContentHashSQLScopesCacheIdentity
• docker run --rm -v "$PWD:/src" -w /src golang:1.24-bookworm go test ./config ./cli ./mcp
• go test ./... in golang:1.24-bookworm container with Node installed and tests run as uid 1000