Git-based PR URL detection via gh pr list

[rysweet]

Add a git-based PR URL detection to _session_gather.py gather command. Run gh pr list --head branch on the remote VM and extract PR URL. More reliable than regex on transcript text.

Files to modify:

• src/amplihack/fleet/_session_gather.py

Requirements:

1. Add _detect_pr_url(vm_name, branch) function
2. Run 'gh pr list --head BRANCH --json url --limit 1' via SSH on remote VM
3. Parse JSON output to extract PR URL
4. Fall back to existing regex-based detection if gh command fails
5. Include PR URL in gathered session context
6. All 918+ fleet tests must pass
7. Commit changes on this branch