Add machine-readable startup errors for filesystem directory validation

Filesystem startup failures are currently human-readable only, which makes client-side classification and automated troubleshooting brittle.

Current behavior is insufficient because invalid or inaccessible allowed directories only emit ad hoc stderr strings such as warnings and a final fatal message, but no stable error code or structured payload that a caller can classify.

Why now: the filesystem server is a reference implementation for fail-closed directory access, so startup validation should be as machine-readable as the runtime protocol surface it protects.

Claim-to-codepath map:

• Startup directory validation in src/filesystem/index.ts
• Roots path validation in src/filesystem/roots-utils.ts
• Startup integration tests in src/filesystem/__tests__/startup-validation.test.ts

Requested behavior:

• Fatal startup validation failures should emit one stable machine-readable error shape.
• The error should identify the failure class (for example no_accessible_directories) and include the rejected inputs when safe.
• Existing fail-closed behavior should remain unchanged.

Evidence Packet

• Commit under test: a97aba19eb21
• Runtime environment:
  • OS: Darwin 25.3.0 arm64
  • Node: v22.19.0
  • pnpm: 10.23.0
• Minimal repro:
  1. Start mcp-server-filesystem with only inaccessible or invalid directory arguments.
  2. Observe stderr output and process exit.
  3. Attempt to classify the failure programmatically.
• Expected behavior: one stable error code/payload for startup validation failures.
• Actual behavior: startup failure details are only available as free-form stderr text.

Acceptance Criteria

• Add a machine-readable startup error format for fatal filesystem directory-validation failures.
• Keep the server fail-closed when no accessible directories are available.
• Add focused startup validation tests that assert the structured error output.

Start with startup directory validation in src/filesystem/index.ts and roots path validation in src/filesystem/roots-utils.ts, then run src/filesystem/tests/startup-validation.test.ts. Define one stable machine-readable error shape for fatal validation failures, including the failure class and safe rejected inputs. Done means focused tests assert the structured output while no accessible directories still cause fail-closed startup.