feat: git mount URL validation with branch detection

- Add POST /config-profiles/validate-git-url endpoint:
  - Parses URL using existing parse_git_url utility
  - Suggests corrected URL for browser URLs
  - Runs git ls-remote --heads to verify reachability
  - Lists available branches from remote
  - Supports SSH key for private repos
  - Returns structured response: valid, suggested_url, branches,
    default_branch, error, error_code

- Update frontend GitMountEditor:
  - Add Check button next to URL field with loading state
  - Show validation result: valid (green), suggestion (yellow),
    invalid (red)
  - Suggestion includes Use this button to apply corrected URL
  - Branch field becomes dropdown when URL is validated,
    populated with remote branches
  - Mappings section disabled until URL is validated
  - Shows hint: Validate the URL first

- Quality gates: pytest (218 passed, 6 pre-existing),
  tsc --noEmit (clean)

openspec/changes/git-mount-url-validation/.openspec.yaml

@@ -0,0 +1,4 @@
+name: git-mount-url-validation
+status: completed
+type: feat
+priority: high