filesystem: lock structured-content response shape for text tools

[davidahmann]

Problem

Filesystem tools expose structured content for clients, but the repo still relies on ad hoc per-tool shaping and narrow tests. That makes schema drift easy to reintroduce and hard to validate automatically.

Why now

Structured content is one of the server's clearest machine-readable artifact surfaces. The filesystem server is also used heavily for client contract testing, so its response shape should be stable and explicit.

Expected behavior

Filesystem tools that declare outputSchema: { content: string } should emit one stable structured-content object shape, with representative regression coverage for file and directory cases.

Claim-to-codepath map

• src/filesystem/index.ts
• src/filesystem/lib.ts
• src/filesystem/__tests__/structured-content.test.ts
• src/everything/tools/get-structured-content.ts

Validation requirements

• Reduce per-tool schema shaping drift through a shared helper or equivalent normalization path.
• Add representative regression coverage for filesystem text-tool responses.
• Keep non-text structured-content surfaces unchanged unless they share the same contract.