feat(tools): Add btw_tool_files_patch for diff-style file editing

[gadenbuie]

Closes #190

Summary

Adds a new files tool, btw_tool_files_patch(), that applies a structured diff-style patch envelope to make coordinated changes across multiple files in a single call. One envelope can add, update, delete, and rename files; every operation is validated (parser, path safety, filesystem preconditions, hunk matching) before any file is written, so a partial failure leaves the working tree untouched.

Key design choices:

• Hunks must include at least one context or delete line as an anchor. Pure-insert hunks are rejected so every edit has a concrete location; use *** Add File for new files.
• Internal hunk representation is a flat tagged-line list (type = "context"|"delete"|"insert") rather than the spec's separate context/deletes/inserts decomposition; both matching and applying become one-pass operations.
• Returns BtwToolResult rather than BtwFileDiffToolResult because a patch can touch multiple files, and the single-file diff viewer shape doesn't map cleanly.

Verification

withr::with_tempdir({
  writeLines(c("a", "b", "c"), "x.txt")
  patch <- "*** Begin Patch
*** Update File: x.txt
@@
 a
-b
+B
 c
*** End Patch
"
  btw::btw_tool_files_patch(patch)
  readLines("x.txt")  # c("a", "B", "c")
})

Tests cover the parser, syntax/filesystem validators, hunk matcher, applier (add/update/delete/move + atomic failure), and tool-integration snapshots. 69 tests pass via testthat::test_file("tests/testthat/test-tool-files-patch.R").