Add MCP spec-compliant _meta field to tool results

[tbtmuse]

Motivation

I use OpenCode with cachebro and it's been awesome for saving tokens. However, OpenCode has a file write protection feature that ensures files are read before they can be modified (FileTime validation). When using cachebro to read files, OpenCode would still show the error:

"You must read file X before overwriting it. Use the Read tool first"

This happened because OpenCode couldn't detect that cachebro had already read the file.

Proposed solution

Add MCP spec-compliant _meta field to cachebro's read_file and read_files tool results to indicate which files were read:

{
  content: [{ type: text, text: file content }],
  _meta: {
    io.github.glommer.cachebro/files: [/path/to/file.ts]
  }
}

This enables MCP clients like OpenCode to track file reads for validation (e.g., FileTime checks) in a spec-compliant way.

Changes

• Add _meta field to read_file and read_files tools
• Returns files read: { "io.github.glommer.cachebro/files": ["/path"] }
• Add test/mcp-meta.test.ts
• Rename test/smoke.ts to test/smoke.test.ts for Bun conventions since there are now multiple tests (a little quality of life thing)
• Update package.json test script to use auto-discovery (also a little quality of life thing)
• Update CI to run all tests

Testing

All tests pass