[codex] Support compound-term fact extraction

[justinjoy]

Summary

• Add explicit string/term metadata for extracted fact subject/relation/object slots.
• Parse only slots marked kind="term" as structural terms; legacy string extraction remains StringLit.
• Validate explicit terms before persistence to avoid partial source/fact writes on malformed or variable-bearing terms.
• Document the explicit extracted term boundary.

Closes #65

Validation

• python -m pytest -q -> 330 passed, 7 skipped
• git diff --check main..HEAD
• Implementation Skill review loop:
  • Architect/explorer design pass completed.
  • Reviewer found strict schema and partial-write concerns.
  • Implementer addressed both concerns.
  • Architect/Critic final validation found no blocking issues.

Notes

OpenAI strict structured output requires every property to be required; note is now required in the provider schema and should be emitted as "" when unused. The parser remains backward-compatible with legacy missing-note and string-slot payloads.