feat: add apply_patch tool (exec-gated)

docs/tools/apply-patch.md

@@ -0,0 +1,49 @@
+---
+summary: "Apply multi-file patches with the apply_patch tool"
+read_when:
+  - You need structured file edits across multiple files
+  - You want to document or debug patch-based edits
+---
+
+# apply_patch tool
+
+Apply file changes using a structured patch format. This is ideal for multi-file
+or multi-hunk edits where a single `edit` call would be brittle.
+
+The tool accepts a single `input` string that wraps one or more file operations:
+
+```
+*** Begin Patch
+*** Add File: path/to/file.txt
++line 1
++line 2
+*** Update File: src/app.ts
+@@
+-old line
++new line
+*** Delete File: obsolete.txt
+*** End Patch
+```
+
+## Parameters
+
+- `input` (required): Full patch contents including `*** Begin Patch` and `*** End Patch`.
+
+## Notes
+
+- Paths are resolved relative to the workspace root.
+- Use `*** Move to:` within an `*** Update File:` hunk to rename files.
+- `*** End of File` marks an EOF-only insert when needed.
+- Experimental and disabled by default. Enable with `tools.exec.applyPatch.enabled`.
+- OpenAI-only (including OpenAI Codex). Optionally gate by model via
+  `tools.exec.applyPatch.allowModels`.
+- Config is only under `tools.exec` (no `tools.bash` alias).
+
+## Example
+
+```json
+{
+  "tool": "apply_patch",
+  "input": "*** Begin Patch\n*** Update File: src/index.ts\n@@\n-const foo = 1\n+const foo = 2\n*** End Patch"
+}
+```