Extract shared git-root resolver for ~/.claude.json project key lookups

[bguidolim]

Context

PR #297 fixed MCPServerCheck falsely reporting MCP servers as "not registered" in subdirectory projects. The root cause: Claude CLI keys local-scope MCP servers in ~/.claude.json by the git root, but ProjectDetector.findProjectRoot() can return a subdirectory (when CLAUDE.local.md is found before .git/).

#297 added a walk-up loop in MCPServerCheck.check() to bridge the gap. However, the same mismatch exists in other call sites, and the fix should be shared rather than duplicated.

Remaining issues

1. ConfigurationDiscovery has the same bug

Export/ConfigurationDiscovery.swift:165-167 does the same direct projects[projectRoot.path] lookup. mcs export from a subdirectory project silently misses project-scoped MCP servers.

2. Walk-up logic should be a shared utility

The walk-up-to-git-root logic in MCPServerCheck should be extracted (e.g., ProjectDetector.findGitRoot(from:) or a helper that probes ~/.claude.json project keys). Both MCPServerCheck and ConfigurationDiscovery should use it.

3. Missing tests for walk-up behavior

PR #297 added no tests. The walk-up loop has non-trivial termination logic (.git/ sentinel + / guard). Per CLAUDE.md, integration tests are mandatory for doctor check changes. Minimum:

• Subdirectory project root finds server keyed at git root → .pass
• Walk-up stops at .git/ boundary → doesn't escape the repo
• Regression: projectRoot == gitRoot still works

4. Minor: candidate: URL? should be plain URL

deletingLastPathComponent() always returns a non-nil URL. The optional wrapper is unnecessary.

Why this matters

Command	Reads ~/.claude.json directly?	Affected?
mcs doctor	Yes (MCPServerCheck)	Fixed by #297
mcs export	Yes (ConfigurationDiscovery)	Still broken
mcs sync	No (delegates to claude CLI)	Not affected
mcs pack remove	No (delegates to claude CLI)	Not affected

Refs

• fix: MCPServerCheck false negative in subdirectory projects #297