fix(app): keep nested directory for local sessions

[Dante-dan]

Issue for this PR

Closes #51191

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

The Web app resolved a new Local session to the Git project's canonical root, even when the selected directory was a subfolder. Keep the selected directory for Local sessions so its .opencode/skills can be discovered. Creating a new worktree still uses the project root.

How did you verify your code works?

• bun test --conditions=solid --preload ./happydom.ts ./src/new-session (9 passed)
• bun typecheck from packages/app (passed)
• Root lint passed. Full root check stopped in an unrelated Astro package because its telemetry tried to write to ~/Library/Preferences/astro, which this environment denies.
• The production session-entry benchmark was attempted before the change; its app build exceeded the configured 120-second web-server startup limit, so no timing comparison was available.

Screenshots / recordings

No visual UI change.

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR

[github-actions]

Thanks for your contribution!

This PR doesn't have a linked issue. All PRs must reference an existing issue.

Please:

1. Open an issue describing the bug/feature (if one doesn't exist)
2. Add Fixes #<number> or Closes #<number> to this PR description

See CONTRIBUTING.md for details.

[github-actions]

The following comment was made by an LLM, it may be inaccurate: