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.

tools/rl_training_tool.py

@@ -169,7 +169,7 @@ def _scan_environments() -> List[EnvironmentInfo]:
             continue
         
         try:
-            with open(py_file, "r") as f:
+            with open(py_file, "r", encoding="utf-8") as f:
                 tree = ast.parse(f.read())
             
             for node in ast.walk(tree):
@@ -333,7 +333,7 @@ async def _spawn_training_run(run_state: RunState, config_path: Path):
         
         # File must stay open while the subprocess runs; we store the handle
         # on run_state so _stop_training_run() can close it when done.
-        api_log_file = open(api_log, "w")  # closed by _stop_training_run
+        api_log_file = open(api_log, "w", encoding="utf-8")  # closed by _stop_training_run
         run_state.api_log_file = api_log_file
         run_state.api_process = subprocess.Popen(
             ["run-api"],
@@ -356,7 +356,7 @@ async def _spawn_training_run(run_state: RunState, config_path: Path):
         # Step 2: Start the Tinker trainer
         logger.info("[%s] Starting Tinker trainer: launch_training.py --config %s", run_id, config_path)
         
-        trainer_log_file = open(trainer_log, "w")  # closed by _stop_training_run
+        trainer_log_file = open(trainer_log, "w", encoding="utf-8")  # closed by _stop_training_run
         run_state.trainer_log_file = trainer_log_file
         run_state.trainer_process = subprocess.Popen(
             [sys.executable, "launch_training.py", "--config", str(config_path)],
@@ -397,7 +397,7 @@ async def _spawn_training_run(run_state: RunState, config_path: Path):
         
         logger.info("[%s] Starting environment: %s serve", run_id, env_info.file_path)
         
-        env_log_file = open(env_log, "w")  # closed by _stop_training_run
+        env_log_file = open(env_log, "w", encoding="utf-8")  # closed by _stop_training_run
         run_state.env_log_file = env_log_file
         run_state.env_process = subprocess.Popen(
             [sys.executable, str(env_info.file_path), "serve", "--config", str(config_path)],
@@ -777,7 +777,7 @@ async def rl_start_training() -> str:
     if "wandb_name" in _current_config and _current_config["wandb_name"]:
         run_config["env"]["wandb_name"] = _current_config["wandb_name"]
     
-    with open(config_path, "w") as f:
+    with open(config_path, "w", encoding="utf-8") as f:
         yaml.dump(run_config, f, default_flow_style=False)
     
     # Create run state
@@ -1206,7 +1206,7 @@ async def rl_test_inference(
             stderr_text = "\n".join(stderr_lines)
             
             # Write logs to files for inspection outside CLI
-            with open(log_file, "w") as f:
+            with open(log_file, "w", encoding="utf-8") as f:
                 f.write(f"Command: {cmd_display}\n")
                 f.write(f"Working dir: {TINKER_ATROPOS_ROOT}\n")
                 f.write(f"Return code: {process.returncode}\n")
@@ -1238,7 +1238,7 @@ async def rl_test_inference(
                 # Parse the output JSONL file
                 if output_file.exists():
                     # Read JSONL file (one JSON object per line = one step)
-                    with open(output_file, "r") as f:
+                    with open(output_file, "r", encoding="utf-8") as f:
                         for line in f:
                             line = line.strip()
                             if not line: