fix(framework): point skill README symlinks to SKILL.md

[alexsoyes]

🎯 What & why

Replace direct per-skill README files with local symlinks to SKILL.md so the skill file remains the canonical source and no standalone README prose is maintained.

Plan: aidd_docs/tasks/2026_06/2026_06_26_remove-skill-readmes/plan.md

🛠️ How it works

• Converts all direct plugins/*/skills/*/README.md entries to README.md -> SKILL.md symlinks.
• Updates skill generation so plugin-source skill creation seeds README.md -> SKILL.md immediately after writing SKILL.md.
• Keeps host-project skill generation focused on the target tool's SKILL.md and does not require README files.
• Makes catalog generation skip direct skill README symlinks while keeping nested asset/template README files.
• Updates contributor docs to describe the local symlink convention.

🧪 How to verify

• pnpm exec lefthook run pre-commit --all-files --force
• one-off assertion that every direct plugins/*/skills/*/README.md is a symlink to SKILL.md
• git diff --check
• npx --yes @ai-driven-dev/cli@4.6.1 framework build --source . --target codex --out "$(mktemp -d)" --flat

⚠️ Heads-up

No dedicated repository validator script is added; the durable behavior is seeded by the skill generator at creation time.