[BUG] Duplicate slash commands when running inside a git worktree

Bug Description

Project-level slash commands (.claude/commands/*.md) appear twice in the command list when Claude Code is running inside a git worktree (non-root). From the root worktree, they appear only once as expected.

Steps to Reproduce

# 1. Create a repo with a project slash command
mkdir root && cd root && git init
mkdir -p .claude/commands
echo 'Print "hello world" to the user.' > .claude/commands/hello-world.md
git add -A && git commit -m "test command"

# 2. Create a worktree
git worktree add ../worktrees/test-01 -b test-01

# 3. Launch Claude Code from the worktree
cd ../worktrees/test-01
claude

# 4. Type "/" to list available slash commands

Expected Behavior

/hello-world should appear once in the command list.

Actual Behavior

/hello-world appears twice:

/hello-world            Print "hello world" to the user. (project)
/hello-world            Print "hello world" to the user. (project)

Root Cause (suspected)

When inside a worktree, Claude Code resolves two different filesystem paths for project commands:

1. The worktree's working directory: .../worktrees/test-01/.claude/commands/
2. The root repository's working directory: .../root/.claude/commands/

Both paths contain the same .claude/commands/hello-world.md (same git content, different filesystem locations). Claude Code loads commands from both without deduplicating.

Key git paths from the worktree:

• git rev-parse --show-toplevel → .../worktrees/test-01 (the worktree)
• git rev-parse --git-common-dir → .../root/.git (points back to root repo)

It seems like Claude Code follows --git-common-dir back to the root repo's .claude/commands/ in addition to the worktree's own copy.

Environment

• OS: Linux (WSL2)
• Claude Code: 2.1.52
• Git worktree setup with separate directories for root and worktrees