docs: add agent bootstrap guides

[f0rr0]

Summary

• Add a root AGENTS.md that explains the repo intent, structure, toolchain, build/check workflow, release boundaries, and agent operating rules.
• Add nested AGENTS.md guides for native, WASIX, React Native/mobile, extensions, and release tooling risk areas.
• Add repo-local agent skills for native-track, release-readiness, mobile-smoke, extension-artifact, and docs-product workflows.

Validation

• git diff --check
• git diff --cached --check
• scoped markdownlint-cli2 on the new AGENTS.md and SKILL.md files
• custom SKILL.md frontmatter/shape validation
• ASCII scan for new files

Notes

• The official skill validator could not run in this shell because PyYAML is not installed, so I used a local read-only frontmatter validator instead.