Add local embedding backend with shared routing and settings

[humandebri]

Summary

• Add a shared embedding backend selector with api as the default and BAAI/bge-m3 as the local option.
• Route query, insert, search, ask-ai, Python, tools, and TUI flows through the shared embedding facade.
• Add kinic-cli embed for embedding diagnostics and prefs set-embedding-backend / TUI settings controls for the shared preference.
• Create new memories at 1024 dimensions, record the backend id for new inserts, and fail explicitly when a selected embedding vector does not match an existing memory dimension.
• Add local chunking and FastEmbed-backed local inference with cache and chunking overrides.

Notes

• Local BAAI/bge-m3 downloads/cache data under $HOME/.cache/kinic-cli/embeddings by default.
• Older non-1024 memories must be reset or reindexed before local search/insert can use them.
• If the shared config directory cannot be loaded, embedding-backed commands now fail instead of silently picking a fallback backend.

Tests

• Added coverage for backend normalization, local config validation, preferences output/mutations, capability JSON, TUI settings projection, and dimension guards.