fix(task-management): support git worktrees in router root detection

[discord9]

Description

Fix task-management router project root detection for git worktrees.

I hit this while using OAC from a real git worktree. In that setup, .git is a file rather than a directory. The router only checked for .git as a directory, so project root detection failed inside the worktree. Because the script also runs with set -e, the fallback path could exit early, which made commands like status and validate return no output.

This change:

• treats .git as either a directory or a file when detecting the project root
• returns success from the fallback path so the router does not exit early under set -e
• restores task-management CLI behavior inside git worktrees

Type of Change

• Bug fix

Checklist

• Tests pass locally
• Documentation updated (if needed)
• Follows CONTRIBUTING.md

Testing

Validated manually in both a normal repo checkout and a real git worktree where the issue was originally observed.

Commands tested:

• bash .opencode/skills/task-management/router.sh status
• bash .opencode/skills/task-management/router.sh validate

Before this change, the worktree case could exit with no output.
After this change, the router correctly finds .tmp/tasks and returns task status/validation results.