feat: add phase 1-2 linear integration

[ssunbles]

Summary

• add new ductor_bot.integrations.linear package with async Linear GraphQL client (aiohttp)
• add Linear models/config and command handlers (/tasks, /task) with button callbacks
• implement Phase 2 AI intake flow for /create via OpenAI/Anthropic/passthrough providers
• add draft confirmation callbacks (linear:draft:confirm|edit|cancel) and orchestrator callback routing
• wire intake config into AgentConfig and callback dispatch in Telegram/shared routers
• add/extend integration tests for intake and callbacks

Verification

• .venv/bin/ruff format . && .venv/bin/ruff check .
• .venv/bin/mypy ductor_bot/integrations/linear/
• .venv/bin/pytest tests/integrations/linear/ -v
• .venv/bin/pytest tests/messenger/telegram/test_app.py tests/messenger/test_multi.py -q