[FEATURE] Support bare repository setup for worktree handling (`claude --worktree`)

Preflight Checklist

• [x] I have searched existing requests and this feature hasn't been requested yet
• [x] This is a single feature request (not multiple features)

Problem Statement

The current behavior of claude-code --worktree expects the working directory is a (recursively) subdirectory of a non-bare repository itself or a worktree of such. I strongly believe it's the root cause of #27436 and #27439.

A bare repository with worktrees is a valid setup, if not commonplace. It's reasonable to take it into account as well.

Proposed Solution

1. The directory under which worktrees are created should be configurable with an executable path or a shell script, regardless of whether it's bare or non-bare setup.

• If not configured:
• non-bare setup: Defaults to $GIT_COMMON_DIR/../.claude/worktrees and creates a worktree under it.
• bare setup: Fails
• If it's configured
• Execute the configured executable or shell script, providing a relevant context (commit-ish, branch name, session id, etc). The spawned process creates a worktree and prints a JSON object to stdout, specifying either a path to the created worktree or an error reason.

1. Respect all Git-specific environment variables related to worktree creation, like GIT_DIR and GIT_COMMON_DIR (I've verified it already does).
2. Don't mess with anything else Git-related. I found that the claude --worktree modifies $GIT_COMMON_DIR/config. See https://github.com/anthropics/claude-code/issues/27474.

Alternative Solutions

Unable to devise workarounds

Priority

Critical - Blocking my work

Feature Category

CLI commands and flags

Use Case Example

As I described above, having worktrees with a bare repo is completely valid setup.

Additional Context

_No response_