fix(rules): classify semantic stem parse refusals as schema-valid

[pablontiv]

What

Classify semantic .stem parse refusals as schema-valid while preserving yaml-valid for YAML syntax failures. Populate field metadata and remove absolute filesystem paths from public stem_health messages.

Related issue

Closes #199

Why

ParseStem previously returned undifferentiated text errors, so stem health mislabeled semantic schema refusals as YAML failures, omitted the affected field, and exposed absolute local or CI paths.

How

• Add an internal typed parse error carrying check, field, path, reason, and optional cause.
• Project typed metadata with errors.As in stem health without parsing strings.
• Preserve contextual paths for direct error consumers while keeping public diagnostic reasons path-free.
• Document the new schema-valid category and record the contract in ADR 0004.
• Add end-to-end regressions for null fields, malformed YAML, and unsupported versions.

Checklist

• Tests pass (go test ./... -race)
• Code is clean (go vet ./... via just check/golangci-lint and build gates)
• Changes are documented if user-facing
• Linked to its issue with a closing keyword