merge upstream/main: incorporate channel refactor, launcher, and modernizations
Resolve 65 file conflicts merging upstream/main into local branch.
Key decisions:
- Accept upstream's channel interface refactoring (clean Channel interface)
- Accept upstream's Cobra CLI restructuring, Launcher TUI, armv6 support
- Accept upstream's HTTP client caching, resp.Body leak fix, regex precompile
- Accept upstream's WeChat/WeCom resource leak fixes and context handling
- Accept upstream's migrate API restructure (NewMigrateInstance pattern)
- Accept upstream's Go 1.22+ syntax modernizations (for range, max/min)
- Preserve local orchestration, plan mode, heartbeat, sandbox, spawn system
- Preserve local SendWithID/MessageSenderWithID for status message dedup
- Preserve local Stream/RPM/functional options in providers
- Rename AGENT.md → AGENTS.md to match upstream template convention
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
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