docs: missing contribution guidelines

[quixoticmonk]

A good contribution guide based on the expectations with the agent-skills format and a possible local tessl skill review .. would be good to include.

[AdamTylerLynch]

PR #88 adds a root CONTRIBUTING.md covering the skill format expectations (frontmatter contract, description-as-trigger, references/ progressive disclosure), an add-a-new-skill checklist including the README/AGENTS/CHANGELOG registration steps, local validation (scripts/validate-structure.sh), and the requested local tessl skill review flow — including TESSL_TOKEN auth for non-interactive use and the fork-PR secrets caveat discovered in #85.