Clean up SQLite references in docs and prompts after removal ...

- docs/index.md: PostgreSQL via asyncpg
- docs/memory.md: drop aiosqlite fallback mention
- docs/sessions.md: replace SqliteSessionStore with PgSessionStore docs
- webclient/docs/architecture.md: PostgreSQL only
- navi/profiles/tool_developer/system_prompt.txt: remove aiosqlite from available imports
- .gitignore: add .codex and *.db
- No remaining sqlite/aiosqlite/db_path references in codebase

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

Update memory docs to reflect pgvector + dedicated embedding backend ...

- Add dedicated embedding backend section (.env variables)
- Add backfill_embeddings script documentation
- Update storage methods: upsert_fact generates embeddings, search_facts
  uses vector search with cosine distance fallback to ILIKE
- Update extractor process: tool calls/results in transcript, source/confidence
- Replace memory_search/memory_forget references with unified memory tool

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

Document pgvector migration in memory system ...

- Add PostgreSQL vs SQLite comparison table

- Document migrate_pgvector.py usage

- Update storage section to mention asyncpg + aiosqlite

Add backend documentation ...

10 markdown files covering all backend subsystems: architecture,
agent loop, tool system, sessions, WebSocket protocol, profiles,
memory, config, and API reference. Structured for AI readability
with a single entry point (docs/index.md) and cross-references.

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