Tighten OKF index reachability validation

[ArunJRK]

Summary

• enforce true strict-mode reachability by traversing links from index.md files instead of accepting any inbound concept link
• add regression coverage for disconnected concept islands plus positive reachability cases
• clarify read-only /okf query behavior, source-selection guardrails for init/update, and Codex-friendly triggers/metadata
• update the dogfood self-bundle and README to reflect the validator and agent-skill wording

Verification

• python3 /Users/arunjrk/.codex/skills/.system/skill-creator/scripts/quick_validate.py okf -> Skill is valid!
• python3 -m pytest okf/scripts/tests -q -> 56 passed
• python3 okf/scripts/validate.py okf/resources/example-bundle --strict -> 8 file(s), 0 error(s), 0 lint(s)
• python3 okf/scripts/validate.py okf/resources/self-bundle --strict -> 22 file(s), 0 error(s), 0 lint(s)
• disconnected a.md <-> b.md island repro now reports both concepts as not reachable from any index.md