feat(oss-readiness): add harness-agnostic public release skill

[bntvllnt]

Summary

• add a new oss-readiness skill with a router, references, and scaffold templates for OSS/public release work
• make the skill harness-agnostic by using AGENTS.md as the canonical instructions file and treating harness-specific files as optional aliases
• replace repo-specific maintainer/contact defaults in skill templates with placeholders and add missing README scaffolding support
• document the new skill in the repo README, skills overview, llms.txt, and llms-full.txt

Validation

• git diff --check -- README.md docs/skills-overview.md llms.txt llms-full.txt oss-readiness
• manual portability audit: no hardcoded maintainer links inside oss-readiness/ (except install path in oss-readiness/README.md)
• manual structure audit: oss-readiness/README.md and oss-readiness/templates/README.md exist and the repo skill count matches the README badge