Add worktree-scoped Git execution

[skarim]

Git worktrees share branch refs and objects, but each has its own HEAD, index, working files, and paused Git operations. This PR adds explicit worktree-scoped Git execution: the foundation for independent stacks in separate checkouts and for one stack whose layers live across several worktrees.

Functionality and user impact

• Discover registered worktrees and the branches they hold, including branches reserved by an interrupted rebase. Keep the shared Git directory distinct from each worktree's local Git directory.
• Run Git in the selected checkout without changing the process directory or inheriting another checkout's index/environment. Refuse missing, replaced, or unrelated repository locations instead of executing somewhere else.
• Keep conflict inspection, rebase continuation, and cherry-picks in the correct worktree. Rebases disable automatic stashing and automatic updates of other refs; date-preserving rebases retain their settings through conflicts.

Boundary: this PR does not move stack metadata, add navigation flags, or automatically run stack commands in other worktrees. Shared tracking follows in #528, cross-worktree rebase/sync in #529, and distributed modify in #521. Discovery requires Git 2.36+; this supports existing worktrees, not their creation or removal.

Key areas to review

• internal/git/worktree.go: ForWorktree and gitClient bind execution to a repository/worktree identity. Focus on rejecting a changed Git directory rather than falling back to the caller.
• internal/git/worktree.go: Worktrees, rebaseBranch, and resolveMainWorktree handle ownership during detached rebases and separate Git directories. Existing core.worktree backlinks are used; unknown checkout paths are not guessed.
• internal/git/git.go: configureCommand, rebaseArgs, and runRebaseCommand isolate the environment, set rebase policy, and distinguish a failure to start from a recoverable conflict.
• internal/git/gitops.go: follow RebaseOnto, RebaseContinue, and the conflict/cherry-pick helpers to confirm checkout-sensitive operations stay on the scoped receiver.

Related issues

• Github stack does not persist across worktrees #459
• gh stack rebase silently succeeds when git rebase --onto fails to start (in branch checked out in another worktree) #35
• sync might not work with worktree #87

*Part 1 of the 4-PR split of #520

[Copilot]

Copilot review overview

🔵 Needs a closer look

The broad, cross-platform changes to Git execution identity and recovery behavior warrant final human validation.

Review effort: Balanced
Findings: None

What changed in this PR

Adds worktree-scoped Git execution while preserving repository identity and isolating checkout-specific state.

Changes:

• Adds worktree discovery, validation, and scoped Git clients.
• Makes rebase, conflict, and cherry-pick operations receiver-scoped.
• Adds extensive integration coverage for worktree behavior and recovery.

File	Description
internal/​git/​worktree.go	Implements worktree discovery and scoping.
internal/​git/​worktree_test.go	Tests parsing, versions, wrappers, and environment isolation.
internal/​git/​git.go	Adds scoped command execution and rebase policy.
internal/​git/​gitops.go	Routes Git operations through scoped receivers.
internal/​git/​gitops_test.go	Adds linked-worktree integration coverage.
internal/​git/​mock_ops.go	Extends mocks for worktree APIs.

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[Copilot]

Copilot review overview

🟡 Changes recommended

Scoped configuration can redirect operations, and invalid scopes can be silently converted into boolean state.

Review effort: Balanced
Findings: 1 · 1

Open (2)

• Sanitize Git worktree configuration for scoped commands · New
• Propagate scoped receiver validation errors instead of hiding them · New