resolve shell from PATH (fallback /bin/sh); rewrite write tool with lineOffset/byteOffset + delLines/delBytes

README.md

@@ -357,20 +357,28 @@ anything is sent back to the model.
 
 ### `write` tool
 
-- Parameters: `filePath` (string, required; also accepts `path`); `offset`
-  (integer, optional, byte offset); `del_bytes` (integer, optional); `content`
-  (string, required, may be empty).
-- Action: if `offset` and `del_bytes` are both omitted, the entire file is
-  overwritten with `content` (the file and any necessary parent directories
-  are created if missing); otherwise `content` is written at `offset`,
-  optionally deleting `del_bytes` bytes first (splicing prefix + content +
-  suffix and never appending to the end of an existing file). (Also accepted as
-  legacy `write_file`.)
+- Parameters: `filePath` (string, required; also accepts `path`); `content`
+  (string, required, may be empty); `lineOffset` (integer, optional, 1-indexed
+  line number to start writing from — takes precedence over `byteOffset`);
+  `byteOffset` (integer, optional, byte offset — used only when `lineOffset` is
+  not given); `delLines` (integer, optional, number of lines to delete starting
+  at the insertion point — takes precedence over `delBytes`); `delBytes`
+  (integer, optional, number of bytes to delete — used only when `delLines` is
+  not given).
+- Action: if none of `lineOffset`, `byteOffset`, `delLines` and `delBytes` are
+  provided, the entire file is overwritten with `content` (the file and any
+  necessary parent directories are created if missing). Otherwise `content` is
+  written at the resolved line/byte offset, optionally deleting `delLines`
+  lines or `delBytes` bytes first (splicing prefix + content + suffix). Line
+  numbers are 1-indexed and match the `read` tool's numbering; a `lineOffset`
+  past the end of the file appends at the end. (Also accepted as legacy
+  `write_file`; legacy `offset`/`del_bytes` are no longer emitted but the
+  `byteOffset`/`delBytes` byte-based mode covers the same behavior.)
 
 ### `bash` tool
 
 - Parameters: `command` (string, required); `workdir` (string, optional).
-- Action: run `command` via `bash -c` subject to `shell_timeout`
+- Action: run `command` through the system shell (`bash` resolved from `PATH`, falling back to `/bin/sh` if bash is unavailable) via `-c`, subject to `shell_timeout`
   (default `120s`) and return `output + '\nexit: ' + exit_code`. (Also
   accepted as legacy `shell_exec`.)