feat(filesystem): add head/tail support to read_multiple_files

[factspark23-hash]

What

Add optional head and tail parameters to the read_multiple_files tool, allowing line-limited reads across multiple files at once.

Why

The single-file read_text_file already supports head and tail, but read_multiple_files does not. When working with multiple large files (logs, configs, data files), agents often only need the first/last N lines of each.

Currently the only option is to call read_text_file with head/tail N times, which is slower and more verbose.

How

read_multiple_files({
  paths: ["log1.txt", "log2.txt", "log3.txt"],
  tail: 20  // last 20 lines of each file
})

• head — returns only the first N lines of each file
• tail — returns only the last N lines of each file
• Cannot use both simultaneously (throws error, same as read_text_file)
• Omitting both reads full files (backward compatible)

Changes

• src/filesystem/index.ts:
  • Added head and tail to ReadMultipleFilesArgsSchema
  • Updated handler to use headFile/tailFile from lib.ts
  • Updated tool description
  • Added mutual exclusion check (same pattern as read_text_file)