docs: add contribution guidelines

[AdamTylerLynch]

Adds a root CONTRIBUTING.md, as requested in #24.

Covers

• Repository/skill layout and the SKILL.md frontmatter contract (what CI enforces vs house style)
• Skill-writing guidance: the description as the trigger, progressive disclosure into references/, soft cross-references by skill name (skills install individually), verifying assets/ code compiles
• Add-a-new-skill checklist including registration in the product README, AGENTS.md, and CHANGELOG.md — the drift that fix: update terraform docs for provider-docs and provider-test-patterns skills #83 had to clean up manually
• Local validation: scripts/validate-structure.sh, jq, and the requested local Tessl review flow (tessl login / TESSL_TOKEN for non-interactive use), including the note that tessl skill review is deprecated upstream and that fork PRs skip the Tessl job because GitHub withholds secrets from fork-triggered workflows (see fix(ci): use TESSL_TOKEN auth, surface CLI errors, skip review on fork PRs #85)
• PR conventions: atomic PRs, conventional-commit titles, CLA

Closes #24.