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>

Use gemma4 cloud model by default

Comprehensive documentation update ...

- websocket.md: add heartbeat, session_sync, replay_start/end, planning_status,
  plan_ready (with is_subagent), stream_end extra fields, context_compressed summary,
  updated _AgentRun.events replay buffer, corrected reconnect section
- sessions.md: add name, planning_logs fields; message flags (is_plan, is_compression,
  is_summary, thinking); set_name store op; debug endpoints section
- api.md: full rewrite — add generate-name, planning, file download endpoints;
  all missing WS events; correct message field table with is_plan/is_compression
- tools.md: update user tools list with all current tools
- index.md: fix profiles list (smart_home → developer)
- CLAUDE.md: add Documentation section with table of all docs files
- NAVI.md: add architecture.md entry, improve websocket description

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

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>