feat: add Deepgram keyterms config

[surfdude75]

Summary

• Add voice.deepgramKeyterms config for Deepgram recognition hints
• Send configured terms as repeated keyterm query parameters in the Deepgram WebSocket URL
• Document the setting and add changelog entry
• Add tests for config loading and URL generation

Testing

• npx bun test — 274 pass, 0 fail

Notes

• npx bun run check currently fails during typecheck on existing unrelated errors in extensions/voice.ts and extensions/voice/tts-deepgram.ts.
• Markdown lint was attempted, but the repository currently has many pre-existing markdownlint issues unrelated to this change.