feat(embedding): add opt-in Nomic task prefixes

[EyJunge1]

Summary

• Implements feat: Add optional task-specific prefixes for nomic-embed-text-v1.5 #98 with an opt-in embeddingUseTaskPrefixes config flag (default false).
• Applies Nomic-style search_document: / search_query: prefixes at store and query call sites when enabled.
• Documents that existing memories should be re-indexed after enabling so store and query vectors stay aligned.

Test plan

• bun test tests/embedding-task-prefixes.test.ts tests/config.test.ts (32 pass)
• Confirm default behavior is unchanged with embeddingUseTaskPrefixes unset/false (default false + prefix helper leaves text unchanged when disabled)
• Confirm document/query prefixes are applied when enabled (search_document: / search_query: unit coverage; call sites pass { task: "document" | "query" })
• Confirm README/config guidance mentions the re-index requirement after enabling

[tickernelz]

Merged into main as part of the newest-to-oldest integration batch and scheduled for the v2.22.0 release.

The combined four-PR tree was verified with:

• bun install --frozen-lockfile
• cd web && bun install --frozen-lockfile
• bun run typecheck
• bun run build (including the React/Vite production bundle)
• bun test — 287 passed / 0 failed / 631 assertions
• git diff --check

The pushed main tree exactly matches the locally tested synthetic merge tree.

[tickernelz]

Released in v2.22.0. The npm registry artifact was downloaded, checksum-verified, installed with lifecycle scripts disabled, and passed native dependency, libSQL vector, package entrypoint, and bundled web UI checks.