feat: parallel task execution with git worktrees in main panel

[Drilmo]

Summary

RFC and initial design for bringing parallel task execution with git worktrees to the main extension panel (ClineProvider).

Related Issue: #5640

Context

Currently, the main sidebar panel executes tasks strictly sequentially:

• Users must wait for one task to complete before starting another
• The Agent Manager already supports parallel execution with worktrees
• This feature would bring that capability to the main panel

Changes

• Added RFC document at .github/docs/parallel-tasks-worktree-extension.md
• Outlines technical design, key components, and migration path

Status

🚧 Draft - This PR is for discussion and design review. Implementation will follow once the approach is validated.

Key Design Points

1. TaskManager: New abstraction to manage multiple active tasks
2. WorktreeManager: Reuse from Agent Manager (shared code)
3. UI Changes: Task tabs, switcher, merge assistant
4. Migration: Phased approach to maintain compatibility

Open Questions

• Rate limiting strategy across parallel tasks?
• Opt-in vs default for parallel mode?
• UI/UX for multiple active tasks?
• Session persistence on restart?

References

• Agent Manager implementation: src/core/kilocode/agent-manager/
• ClineProvider: src/core/webview/ClineProvider.ts
• PR Parallel agents #3355, Add Agent Manager for multi-agent orchestration #4151, Agent manager: Allow spawning multiple non-interactive agent versions on git worktrees #4380, Migrate Agent Manager from CLI to agent-runtime architecture #5263 (Agent Manager history)

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