feat: Add rebase and fanout commands for multi-worktree synchronization

[ZHLX2005]

Summary

Adds rebase and fanout commands for synchronizing work across multiple git worktrees.

ccswitch rebase

Commit and rebase changes from any worktree onto the current branch.

• Works with all git worktrees (not just sessions)
• Color-coded status: 🟡 uncommitted | 🟢 ahead | ⚪ behind
• Auto-abort on conflicts

ccswitch fanout

Propagate current branch commits to all other worktrees.

• Safety checks: no uncommitted changes, no worktrees ahead
• Auto-abort on conflicts
• Ideal for syncing all feature branches with core branch

Files Added

• cmd/rebase.go, cmd/fanout.go
• internal/git/commit.go, internal/git/rebase.go

Files Modified

• cmd/root.go
• internal/git/repository.go
• internal/session/manager.go