codebase: add encoding='utf-8' to all bare open() calls (PLW1514)

Closes the last Python-on-Windows UTF-8 exposure by making every
text-mode open() call explicit about its encoding.

Before: on Windows, bare open(path, 'r') defaults to the system
locale encoding (cp1252 on US-locale installs).  That means reading
any config/yaml/markdown/json file with non-ASCII content either
crashes with UnicodeDecodeError or silently mis-decodes bytes.

After: all 89 affected call sites in production code now pass
encoding='utf-8' explicitly.  Works identically on every platform
and every locale, no surprise behavior.

Mechanical sweep via:
  ruff check --preview --extend-select PLW1514 --unsafe-fixes --fix     --exclude 'tests,venv,.venv,node_modules,website,optional-skills,               skills,tinker-atropos,plugins' .

All 89 fixes have the same shape: open(x) or open(x, mode) became
open(x, encoding='utf-8') or open(x, mode, encoding='utf-8').  Nothing
else changed.  Every modified file still parses and the Windows/sandbox
test suite is still green (85 passed, 14 skipped, 0 failed across
tests/tools/test_code_execution_windows_env.py +
tests/tools/test_code_execution_modes.py + tests/tools/test_env_passthrough.py +
tests/test_hermes_bootstrap.py).

Scope notes:
  - tests/ excluded: test fixtures can use locale encoding intentionally
    (exercising edge cases).  If we want to tighten tests later that's
    a separate PR.
  - plugins/ excluded: plugin-specific conventions may differ; plugin
    authors own their code.
  - optional-skills/ and skills/ excluded: skill scripts are user-authored
    and we don't want to mass-edit them.
  - website/ and tinker-atropos/ excluded: vendored / generated content.

46 files touched, 89 +/- lines (symmetric replacement).  No behavior
change on POSIX or on Windows when the file is ASCII; bug fix on
Windows when the file contains non-ASCII.

agent/curator.py

@@ -1607,7 +1607,7 @@ def _run_llm_review(prompt: str) -> Dict[str, Any]:
         # terminal. The background-thread runner also hides it; this
         # belt-and-suspenders path matters when a caller invokes
         # run_curator_review(synchronous=True) from the CLI.
-        with open(os.devnull, "w") as _devnull, \
+        with open(os.devnull, "w", encoding="utf-8") as _devnull, \
              contextlib.redirect_stdout(_devnull), \
              contextlib.redirect_stderr(_devnull):
             conv_result = review_agent.run_conversation(user_message=prompt)

agent/model_metadata.py

@@ -754,7 +754,7 @@ def _load_context_cache() -> Dict[str, int]:
     if not path.exists():
         return {}
     try:
-        with open(path) as f:
+        with open(path, encoding="utf-8") as f:
             data = yaml.safe_load(f) or {}
         return data.get("context_lengths", {})
     except Exception as e:
@@ -776,7 +776,7 @@ def save_context_length(model: str, base_url: str, length: int) -> None:
     path = _get_context_cache_path()
     try:
         path.parent.mkdir(parents=True, exist_ok=True)
-        with open(path, "w") as f:
+        with open(path, "w", encoding="utf-8") as f:
             yaml.dump({"context_lengths": cache}, f, default_flow_style=False)
         logger.info("Cached context length %s -> %s tokens", key, f"{length:,}")
     except Exception as e:
@@ -800,7 +800,7 @@ def _invalidate_cached_context_length(model: str, base_url: str) -> None:
     path = _get_context_cache_path()
     try:
         path.parent.mkdir(parents=True, exist_ok=True)
-        with open(path, "w") as f:
+        with open(path, "w", encoding="utf-8") as f:
             yaml.dump({"context_lengths": cache}, f, default_flow_style=False)
     except Exception as e:
         logger.debug("Failed to invalidate context length cache entry %s: %s", key, e)

agent/nous_rate_guard.py

@@ -144,7 +144,7 @@ def nous_rate_limit_remaining() -> Optional[float]:
     """
     path = _state_path()
     try:
-        with open(path) as f:
+        with open(path, encoding="utf-8") as f:
             state = json.load(f)
         reset_at = state.get("reset_at", 0)
         remaining = reset_at - time.time()

agent/shell_hooks.py

@@ -617,7 +617,7 @@ def _locked_update_approvals() -> Iterator[Dict[str, Any]]:
             save_allowlist(data)
         return
 
-    with open(lock_path, "a+") as lock_fh:
+    with open(lock_path, "a+", encoding="utf-8") as lock_fh:
         fcntl.flock(lock_fh.fileno(), fcntl.LOCK_EX)
         try:
             data = load_allowlist()