Make filesystem directory-tree output ordering deterministic

Problem

Directory tree responses can vary by host filesystem enumeration order, causing unstable output for the same directory across runs.

Why now

The filesystem server is a reference implementation; deterministic output is required for reproducible comparisons and contract tests.

Expected behavior

For the same directory contents and exclude patterns, directory_tree should return a stable ordering across repeated runs and platforms.

Claim-to-codepath map

• src/filesystem/index.ts
• src/filesystem/lib.ts
• src/filesystem/__tests__/directory-tree.test.ts
• src/filesystem/__tests__/structured-content.test.ts

Validation requirements

• Enforce deterministic sorting in recursive tree traversal.
• Add test assertions for stable sibling ordering and nested ordering.
• Run filesystem package tests.

Evidence packet

• Commit under test: $(git -C /Users/davidahmann/Projects/agent-ecosystem/mcp-servers rev-parse HEAD)
• Runtime: Node.js + vitest on macOS
• Minimal repro: call directory_tree multiple times over same fixture and compare ordering of siblings.

Start with src/filesystem/index.ts and src/filesystem/lib.ts to trace directory_tree's recursive traversal and current filesystem enumeration. Review directory-tree.test.ts and structured-content.test.ts, then run the filesystem package tests. Done means repeated calls over the same fixture produce stable sibling and nested ordering across the stated cases.