bug fix: support Windows Docker paths, Gemini model defaults, and session stability

[liuh886]

This PR fixes several critical issues encountered when running ductor on Windows with Docker sandboxing and improves the Gemini user experience.

1. Fix Gemini Model List: Added DEFAULT_GEMINI_MODELS as a fallback. Previously, if dynamic discovery failed, the model list would remain empty, preventing users from selecting models (including auto). [Bug]: Gemini model list is wiped and remains empty if dynamic discovery fails #34
2. Windows Docker Compatibility: (Referencing commit d9063a4) Fixed pathing issues where Windows-style backslashes were being passed to Docker's chdir, causing sub-agent startup failures. [Bug]: Windows Docker + Gemini runtime issues in dev build #33
3. Session Stability: (Referencing commit 0aac267) Fixed an issue where specific trigger words like @Botfather could cause session errors. [Bug]: Session error - Failed to start Gemini Cli due to "@BotFather" in agent_tools #32

Testing Done

• Verified that /model now correctly shows Gemini models even when dynamic discovery is unavailable.
• Verified sub-agents start correctly in Docker on a Windows host.
• Verified that existing sessions are correctly resolved.

[PleasePrompto]

Hey @liuh886, thanks a lot for this PR! Really appreciate you taking the time to dig into these issues and put together fixes.

I can't easily test Windows + Docker + Gemini setups myself, so having someone actually hitting these bugs in the wild and reporting them with concrete details is super valuable.

I've merged your PR and then layered our own fixes on top. Here's what we extended:

• Cache fallback ([Bug]: Gemini model list is wiped and remains empty if dynamic discovery fails #34): We went with a three-tier approach in BaseModelCache — preserve existing disk cache first, then use hardcoded fallback models (in-memory only, never persisted to disk so the next successful discovery replaces them cleanly). This covers both Codex and Gemini, and Codex fallbacks include thinking levels (supported_efforts).
• Windows paths ([Bug]: Windows Docker + Gemini runtime issues in dev build #33): Same fix direction as yours, plus .as_posix() for Path objects in base.py.
• @Botfather ([Bug]: Session error - Failed to start Gemini Cli due to "@BotFather" in agent_tools #32): Identical fix — yours was already merged, ours got auto-skipped during rebase.
• Topic routing ([Bug]: Background task results sometimes return to user DM instead of original group topic #31): Added chat_id/topic_id propagation through the entire async inter-agent chain so results land in the right forum topic.
• .env secrets ([Feature]: Support external API secrets across host and Docker runtimes #30): New feature — ~/.ductor/.env for external API keys, auto-injected into host + Docker execution with mtime-based hot-reload.

Thanks again for the contribution — keep the bug reports coming!