fix: derive paths-only branch from feature directory

[luohui1]

Description

Closes #3026 by deriving the paths-only BRANCH value from the resolved feature directory basename when no explicit SPECIFY_FEATURE is available.

This keeps check-prerequisites --paths-only / -PathsOnly useful for feature-context invocations that rely on .specify/feature.json instead of an active feature branch environment variable.

Split out from #3053 after maintainer feedback to keep each bug fix in its own PR.

Testing

• Tested locally with uv run specify --help
• Ran existing tests with uv sync && uv run pytest
• Tested with a sample project (if applicable)

Additional validation run locally:

• uv run --extra test pytest tests/test_check_prerequisites_paths_only.py -q - 4 passed, 5 skipped
• uvx ruff check tests/test_check_prerequisites_paths_only.py - passed
• git diff --check - passed

Manual test results

Agent: Codex (GPT-5) | OS/Shell: Windows 11 / PowerShell 7.5.4

Command tested	Notes
.specify/scripts/powershell/check-prerequisites.ps1 -Json -PathsOnly	Covered by regression test for non-empty BRANCH fallback from feature directory basename.

AI Disclosure

• I did not use AI assistance for this contribution
• I did use AI assistance (describe below)

This PR was implemented by Codex (GPT-5) acting as an AI coding agent on behalf of @luohui1. The agent read the repository contribution/security/code-of-conduct docs and issue templates, inspected the relevant code paths, implemented the changes, ran the validation listed above, and prepared this PR description.