fix: add error handling to 8 tools with unguarded async handlers

[TerminalGravity]

8 tools had no try/catch around their async handlers. If git wasn't available, state files were corrupt, or directories were missing, they'd throw unhandled errors and crash the MCP server.

Now they return user-friendly error messages (e.g. ❌ checkpoint failed: ...) instead of crashing.

Tools fixed: audit_workspace, check_patterns, checkpoint, search_contracts, sequence_tasks, session_health, sharpen_followup, what_changed

[TerminalGravity]

Good defensive addition. The try/catch wrappers ensure MCP tools return error content instead of crashing the server process.

One concern: this PR still uses the broken run() calls (e.g., what-changed.ts line 13 still has run(\git diff ...`)`). This should be rebased on top of #220 after it merges, otherwise you're wrapping error handlers around calls that silently fail anyway.

Also, the pattern could be DRY'd with a wrapper:

function safeTool(fn: () => Promise<ToolResult>): Promise<ToolResult> {
  try { return await fn(); }
  catch (err) { return { content: [{ type: 'text', text: \`❌ \${err}\` }] }; }
}

Not blocking, but worth considering to avoid the indent creep across 8 files.