feat(tools): implement Edit and Write tools with deterministic patches (M3 complete)

This commit implements the complete M3 milestone (Edit & Write tools) including:

Write tool:
- Creates new files with parent directory creation
- Overwrites existing files safely
- Simple and straightforward implementation

Edit tool:
- Exact string replacement with uniqueness enforcement
- Detects ambiguous matches (multiple occurrences) and fails safely
- Detects no-match scenarios and fails with clear error
- Automatic backup before modification
- Rollback on write failure (restores from backup)
- Supports multiline string replacements

CLI integration:
- Added `write` subcommand: `owlen write <path> <content>`
- Added `edit` subcommand: `owlen edit <path> <old_string> <new_string>`
- Permission checks for both Write and Edit tools
- Clear error messages for permission denials

Permission enforcement:
- Plan mode (default): blocks Write and Edit (asks for approval)
- AcceptEdits mode: allows Write and Edit
- Code mode: allows all operations

Testing:
- 6 new tests in tools-fs for Write/Edit functionality
- 5 new tests in CLI for permission enforcement with Edit/Write
- Tests verify plan mode blocks, acceptEdits allows, code mode allows all
- All 32 workspace tests passing

Dependencies:
- Added `similar` crate for future diff/patch enhancements

M3 milestone complete! ✅

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

crates/tools/fs/tests/fs_tools.rs

@@ -1,4 +1,4 @@
-use tools_fs::{read_file, glob_list, grep};
+use tools_fs::{read_file, glob_list, grep, write_file, edit_file};
 use std::fs;
 use tempfile::tempdir;
 
@@ -26,4 +26,79 @@ fn grep_finds_lines() {
 
     let hits = grep(root.to_str().unwrap(), "hello").unwrap();
     assert!(hits.iter().any(|(_p, _ln, text)| text.contains("hello")));
+}
+
+#[test]
+fn write_file_creates_new_file() {
+    let dir = tempdir().unwrap();
+    let file_path = dir.path().join("new.txt");
+
+    write_file(file_path.to_str().unwrap(), "new content").unwrap();
+
+    assert_eq!(read_file(file_path.to_str().unwrap()).unwrap(), "new content");
+}
+
+#[test]
+fn write_file_overwrites_existing() {
+    let dir = tempdir().unwrap();
+    let file_path = dir.path().join("existing.txt");
+    fs::write(&file_path, "old content").unwrap();
+
+    write_file(file_path.to_str().unwrap(), "new content").unwrap();
+
+    assert_eq!(read_file(file_path.to_str().unwrap()).unwrap(), "new content");
+}
+
+#[test]
+fn edit_file_replaces_exact_match() {
+    let dir = tempdir().unwrap();
+    let file_path = dir.path().join("test.txt");
+    let original = "line 1\nline 2\nline 3\n";
+    fs::write(&file_path, original).unwrap();
+
+    edit_file(file_path.to_str().unwrap(), "line 2", "modified line 2").unwrap();
+
+    let result = read_file(file_path.to_str().unwrap()).unwrap();
+    assert_eq!(result, "line 1\nmodified line 2\nline 3\n");
+}
+
+#[test]
+fn edit_file_replaces_multiline() {
+    let dir = tempdir().unwrap();
+    let file_path = dir.path().join("test.txt");
+    let original = "line 1\nline 2\nline 3\nline 4\n";
+    fs::write(&file_path, original).unwrap();
+
+    edit_file(file_path.to_str().unwrap(), "line 2\nline 3", "new content").unwrap();
+
+    let result = read_file(file_path.to_str().unwrap()).unwrap();
+    assert_eq!(result, "line 1\nnew content\nline 4\n");
+}
+
+#[test]
+fn edit_file_fails_on_ambiguous_match() {
+    let dir = tempdir().unwrap();
+    let file_path = dir.path().join("test.txt");
+    let original = "duplicate\nsome text\nduplicate\n";
+    fs::write(&file_path, original).unwrap();
+
+    let result = edit_file(file_path.to_str().unwrap(), "duplicate", "changed");
+
+    assert!(result.is_err());
+    let err_msg = result.unwrap_err().to_string();
+    assert!(err_msg.contains("Ambiguous") || err_msg.contains("multiple") || err_msg.contains("occurrences"));
+}
+
+#[test]
+fn edit_file_fails_on_no_match() {
+    let dir = tempdir().unwrap();
+    let file_path = dir.path().join("test.txt");
+    let original = "line 1\nline 2\n";
+    fs::write(&file_path, original).unwrap();
+
+    let result = edit_file(file_path.to_str().unwrap(), "nonexistent", "changed");
+
+    assert!(result.is_err());
+    let err_msg = result.unwrap_err().to_string();
+    assert!(err_msg.contains("not found") || err_msg.contains("String to replace"));
 }