release: v0.39.0 — agent profile editor, API key management, reasoning effort, embedded terminal

New Features:
- Agent Profile Config UI: editable model, provider, base_url, reasoning
  effort, max_turns, system_prompt, personality, and toolsets via new
  Settings tabs (General / Agent / API Keys)
- Backend: GET/POST /api/profile/config endpoint reads/writes config.yaml
  sections and .env API keys
- API Keys editor: masked display, inline edit with auto-save, add/remove keys
  per known provider environment variable
- Reasoning effort: full integration across session creation, switching, and
  chat — clamp_reasoning_effort() validates against model metadata
  (supports xhigh/high/medium/low/minimal per provider)
- Streaming: _ensure_final_response_message() for reliable message history,
  _persist_stream_error() for visible error persistence in sessions
- Embedded terminal: iframe-based ttyd via nginx proxy with control key
  toolbar (Ctrl+C, Ctrl+D, Ctrl+L, etc.) and status indicator
- Provider discovery: expanded model lists for Kimi, DeepSeek, Mistral,
  OpenAI Codex; dynamic OpenRouter free model list; provider sorting
  by configured credentials

Bug Fixes:
- Fix _write_env_changes boolean logic (and -> or) for env var removal
- Updated model names to current API (gpt-5.3-codex, DeepSeek V3.2, etc.)

Tests:
- test_session_reasoning.py: reasoning effort validation
- test_streaming_errors.py: stream error handling
- test_model_resolver.py: +380 lines expanded coverage

~2,900 lines across 15 files (12 modified, 3 new)

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# Author:    Pipo <nesquena+hermes@gmail.com>
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