Windows: agent CLI output in ANSI/GBK codepage crashes subprocess reader threads (fix: errors="replace")  #64

### Environment

- lh-harness 0.1.6 (PyPI), Windows 10 (10.0.26200)
- Python 3.13.14
- Agent backend: `codex` (third-party build whose `--version` output contains GBK bytes on zh-CN Windows)
- Windows "Use Unicode UTF-8 for worldwide language support" **enabled** (system ACP = 65001), so Python's text-mode subprocess defaults to UTF-8

### What happened

`lh-harness run` starts, creates the run directory, enters Round 1, then both
subprocess reader threads die and the manager episode ends in
`UnicodeDecodeError`. The run is then misreported as `provider_network` /
`provider_model_unavailable`:

```
── Round 1/25 ──
  [manager] running...
Exception in thread Thread-6 (_readerthread):
  ...
  File "subprocess.py", line 1615, in _readerthread
    buffer.append(fh.read())
UnicodeDecodeError: 'utf-8' codec can't decode byte 0xb4 in position 0: invalid start byte
Exception in thread Thread-7 (_readerthread):
UnicodeDecodeError: 'utf-8' codec can't decode byte 0xb3 in position 0: invalid start byte
  [manager] failed · 308.8s
Stopped: provider_network
```

### Root cause

`utils/agent_cli.py` validates the agent binary with:

```python
subprocess.run([path, "--version"], capture_output=True, text=True, ...)
```

On zh-CN Windows, agent CLIs (and Windows helpers such as `taskkill`) may emit
ANSI/GBK-encoded output while the harness decodes pipes as UTF-8 (system UTF-8
codepage or `PYTHONUTF8=1`). With two captured pipes this spawns two reader
threads; both crash, the version probe returns nothing, and the failure is
misattributed to the provider.

### Fix

Make every text-mode subprocess call tolerant of non-UTF-8 bytes by adding
`errors="replace"`. Four files (five call sites) on 0.1.6:

```diff
--- a/src/lh_harness/utils/agent_cli.py
+++ b/src/lh_harness/utils/agent_cli.py
@@ def _probe (subprocess.run([path, "--version"], ...)
             capture_output=True,
             text=True,
+            errors="replace",
             timeout=timeout,

--- a/src/lh_harness/model_catalog.py
+++ b/src/lh_harness/model_catalog.py
@@ _codex_app_server_models
         stderr=subprocess.DEVNULL,
         text=True,
+        errors="replace",
         bufsize=1,

--- a/src/lh_harness/utils/process_group.py
+++ b/src/lh_harness/utils/process_group.py
@@ _run_quiet
             capture_output=True,
             text=True,
+            errors="replace",
             timeout=15,

--- a/src/lh_harness/plugins/npm.py
+++ b/src/lh_harness/plugins/npm.py
@@ two subprocess.run call sites
             capture_output=True,
             text=True,
+            errors="replace",
```

(`adapters/deepseek_runner.py` already passes `errors="replace"` — these five
sites just missed it.)

With this patch (plus PR #29 for the control-bus blocker), a full
Manage→Execute→Audit run completes on Windows — verified with `claude_code`
backend, 2 rounds, auditor confirmed against the filesystem independently.

### Two related observations

1. **`doctor` blind spot:** on stock 0.1.6, `lh-harness doctor` prints
   `Doctor result: ready` even though `run` is hard-broken on Windows by the
   POSIX-only control-bus hardening (`O_NOFOLLOW` / `O_DIRECTORY` / `dir_fd`,
   see `supervisor/control_bus.py:127`). PR #29 fixes the run path. It would
   help if `doctor` probed control-bus capability on Windows and warned
   accordingly, since "ready" currently over-promises.

2. **Misattributed abort reason:** when an agent CLI probe dies in a reader
   thread, the run is reported as `provider_network`. The actual cause (local
   subprocess decode crash) only shows in the episode `agent.log`. A stderr
   pattern check for `UnicodeDecodeError` could surface the real reason.

Happy to turn the `errors="replace"` changes into a PR if the maintainers
prefer.