fix(opencode): Use opencode.jsonc when present (#1590)

## Description

Fix OpenCode proxy injection so it respects user configurations that use
the `.jsonc` extension, preventing Headroom from creating a duplicate
`.json` file that overrides it.

Closes #1588

## Type of Change

- [x] Bug fix (non-breaking change that fixes an issue)
- [ ] New feature (non-breaking change that adds functionality)
- [ ] Breaking change (fix or feature that would cause existing
functionality to change)
- [ ] Documentation update
- [ ] Performance improvement
- [ ] Code refactoring (no functional changes)

## Changes Made

- Updated `opencode_config_path` in `paths.py` to check for `.jsonc`
- Updated backup creation in `config.py` to preserve the original
extension

## Testing

<!-- Check what you actually ran, then paste the real command output
below. -->

- [ ] Unit tests pass (`pytest`)
- [ ] Linting passes (`ruff check .`)
- [ ] Type checking passes (`mypy headroom`)
- [ ] New tests added for new functionality
- [x] Manual testing performed

### Test Output

```text
N/A
```

## Real Behavior Proof

- Environment: local headroom dev
- Exact command / steps: creating a dummy
`.config/opencode/opencode.jsonc` and running `headroom wrap opencode`.
- Observed result: Headroom successfully injects into `.jsonc` and
creates a backup named `opencode.jsonc.headroom-backup`.
- Not tested: N/A

## Review Readiness

- [x] I have performed a self-review
- [x] This PR is ready for human review

## Checklist

- [x] My code follows the project's style guidelines
- [x] I have performed a self-review of my code
- [ ] I have commented my code, particularly in hard-to-understand areas
- [ ] I have made corresponding changes to the documentation
- [x] My changes generate no new warnings
- [ ] I have added tests that prove my fix is effective or that my
feature works
- [ ] New and existing unit tests pass locally with my changes
- [ ] I have updated the CHANGELOG.md if applicable

## Screenshots (if applicable)

## Additional Notes

tests/test_cli/test_wrap_opencode.py

@@ -346,7 +346,7 @@ def test_unwrap_opencode_restores_from_backup(
     _set_test_home(monkeypatch, tmp_path)
 
     config_file = tmp_path / ".config" / "opencode" / "opencode.json"
-    backup_file = config_file.with_suffix(".json.headroom-backup")
+    backup_file = config_file.with_name("opencode.json.headroom-backup")
     config_file.parent.mkdir(parents=True, exist_ok=True)
     original = '{"model": "openai/gpt-4o"}'
     config_file.write_text(original)
@@ -361,6 +361,31 @@ def test_unwrap_opencode_restores_from_backup(
     assert config_file.read_text(encoding="utf-8") == original
 
 
+def test_unwrap_opencode_restores_from_backup_jsonc(
+    runner: CliRunner,
+    tmp_path: Path,
+    monkeypatch: pytest.MonkeyPatch,
+) -> None:
+    """Unwrap restores the pre-wrap backup and removes it for jsonc files."""
+    monkeypatch.chdir(tmp_path)
+    _set_test_home(monkeypatch, tmp_path)
+
+    config_file = tmp_path / ".config" / "opencode" / "opencode.jsonc"
+    backup_file = config_file.with_name("opencode.jsonc.headroom-backup")
+    config_file.parent.mkdir(parents=True, exist_ok=True)
+    original = '{\n  // User comment\n  "model": "openai/gpt-4o"\n}'
+    config_file.write_text(original)
+    backup_file.write_text(original)
+
+    with patch.object(wrap_mod, "_stop_local_proxy_for_unwrap", return_value="stopped"):
+        result = runner.invoke(main, ["unwrap", "opencode"])
+
+    assert result.exit_code == 0, result.output
+    assert "Restored prior" in result.output
+    assert not backup_file.exists()
+    assert config_file.read_text(encoding="utf-8") == original
+
+
 def test_unwrap_opencode_strips_blocks_when_no_backup(
     runner: CliRunner,
     tmp_path: Path,