fix(file-tools): make write_file/patch atomic (temp-file + rename) (#35252)

* Inspired by Claude Code: /compress here [N] — boundary-aware 'summarize up to here'

Adds a user-chosen compression boundary to the existing /compress command.
/compress here [N] summarizes everything except the most recent N exchanges
(default 2), which are preserved verbatim — letting the user pick the
compression boundary instead of relying on the automatic token-budget heuristic.

Inspired by Claude Code's Rewind 'Summarize up to here' action (v2.1.139,
Week 20, May 2026): https://code.claude.com/docs/en/whats-new/2026-w20

- hermes_cli/partial_compress.py: pure split/parse helpers + seam-alternation
  guard (shared by CLI and gateway).
- cli.py / gateway/run.py: route 'here [N]' / '--keep N' to partial compression;
  compress only the head, re-append the verbatim tail through the seam guard.
- Preserves message-flow role alternation (seam guard merges any illegal
  user->user / assistant->assistant adjacency).
- Reuses the existing _compress_context session-rotation/lock machinery — no
  changes to the compression core.
- Bare /compress (full) and /compress <focus> behavior unchanged.

Tests: 12 helper unit tests + 5 CLI integration tests + E2E (interleaved
tool-call transcript, degenerate/multimodal seams, real handler path).

* fix(file-tools): make write_file/patch atomic (temp-file + rename)

write_file streamed content straight into the target via `cat > path`, so
a crash, SIGKILL, or truncated pipe mid-write left the file half-written
and corrupt. patch_replace routes through write_file, so it shared the flaw.

Now writes stream into a temp file in the SAME directory and `mv` it over
the target — a real same-filesystem rename, which is atomic on POSIX and on
every terminal backend (local/docker/ssh/modal). A failed write leaves the
original byte-intact and leaks no temp file. The existing file's mode is
preserved across the swap (stat + chmod, GNU/BSD), and content still rides
stdin so there's no ARG_MAX limit. A trap cleans the temp on any error path.

Tests: added TestAtomicWrite (real LocalEnvironment, no mocks) covering
inode-change-on-overwrite, mode preservation, failed-write-leaves-original,
no-temp-leak, special chars, and patch routing. Updated two mocks in
test_file_operations.py that keyed on the literal `cat >` write command to
key on the stdin_data behavioral signal instead. 200 file-tool tests green.

tests/tools/test_file_operations.py

@@ -638,12 +638,14 @@ class TestPatchReplacePostWriteVerification:
         state = {"content": "hello world\n"}
 
         def side_effect(command, stdin_data=None, **kwargs):
-            # Write is `cat > path` — detect by the `>` redirect, NOT just `cat `
-            if command.startswith("cat >"):
-                if stdin_data is not None:
-                    state["content"] = stdin_data
+            # A write is the only call that pipes content over stdin — key
+            # on that behavioral signal rather than the exact write command,
+            # which is an atomic temp-file + mv script (`set -e; ... mv ...`),
+            # not a bare `cat > path`.
+            if stdin_data is not None:
+                state["content"] = stdin_data
                 return {"output": "", "returncode": 0}
-            if command.startswith("cat "):  # read
+            if command.startswith("cat "):  # read / verify
                 return {"output": state["content"], "returncode": 0}
             if command.startswith("mkdir "):
                 return {"output": "", "returncode": 0}
@@ -664,9 +666,8 @@ class TestPatchReplacePostWriteVerification:
         state = {"content": "hello world\n"}
 
         def side_effect(command, stdin_data=None, **kwargs):
-            if command.startswith("cat >"):  # write
-                if stdin_data is not None:
-                    state["content"] = stdin_data
+            if stdin_data is not None:  # write (atomic temp-file + mv script)
+                state["content"] = stdin_data
                 return {"output": "", "returncode": 0}
             if command.startswith("cat "):  # read
                 call_count["cat"] += 1