fix: reject unknown YAML config keys

[Luqueee]

Summary\n\n- decode configuration with strict YAML known-field validation\n- report the unknown key together with the config path\n- preserve defaults for partial and empty configuration files\n- add regression coverage for unknown keys and empty YAML\n\nCloses #15\n\n## Validation\n\n- 165 Go tests passed\n- race tests passed\n- go vet passed\n- gofmt and diff checks passed

[coderabbitai]

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📝 Summary

Summary by CodeRabbit

• Bug Fixes

  • Configuration files now reject unrecognized top-level YAML settings and multiple YAML documents.
  • Malformed YAML errors include the configuration file path for easier troubleshooting.
  • Empty configuration files continue to load successfully while retaining default values.

• Tests

  • Added coverage for unknown settings, multiple documents, malformed trailing content, error paths, and empty configuration behavior.

Walkthrough

Changes

Load now uses strict YAML decoding. Unknown keys, multiple documents, and malformed trailing documents return errors that include the configuration path. Empty YAML input remains valid and preserves default log level and SSH port values.

Configuration loading

Layer / File(s)	Summary
Strict YAML decoding and validation internal/config/config.go, internal/config/config_test.go	Load uses yaml.Decoder with KnownFields(true), accepts empty input, and rejects additional YAML documents. Tests cover unknown keys, empty input, multiple documents, and malformed trailing documents.

Merge Risk: ⚪ Minimal · up to ad917

Configuration loading now rejects unknown fields and trailing YAML documents while preserving defaults for empty or partial configuration. The covered behavior is ready to merge.

🚥 Pre-merge checks | ✅ 4

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title uses the required Conventional Commit format and clearly states the main change: rejecting unknown YAML configuration keys.
Description check	✅ Passed	The description accurately summarizes strict YAML validation, default preservation, regression tests, and validation results. It directly relates to the changeset.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

✨ Finishing Touches

📝 Generate docstrings

• Create stacked PR
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• Create PR with unit tests
• Commit unit tests in branch codex/reject-unknown-config

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[coderabbitai]

Actionable comments posted: 1

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Reviewing files that changed from the base of the PR and between 3995291 and a407f35.

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• internal/config/config.go
• internal/config/config_test.go

Included review availability: Your plan provides up to 8 included reviews per hour; 2 remain after this review.

[Luqueee]

Fixes Applied Successfully\n\nApplied the approved CodeRabbit finding.\n\nChanges:\n- trailing YAML documents are now rejected\n- malformed trailing documents include the config path in the error\n- empty YAML files continue to load defaults\n\nCommit: ad91729\n\nValidation passed: 167 tests, race tests, go vet, format checks, and diff checks.