Add StitchFlow external skill

[yshishenya]

Summary\n- add StitchFlow as an external skill source\n- include the imported skill files under External/stitchflow\n- list StitchFlow in README and manifest metadata\n\n## Validation\n- bun run index.ts validate\n\nValidation still reports the repo's existing warnings for other external skills, plus the same root-directory warning for External/stitchflow because it includes a workflows directory.

[yshishenya]

Adding validation context for maintainers: I kept this PR intentionally minimal (external.yml, README.md, manifest.json, and External/stitchflow/*). Local bun run index.ts validate still reports the repo's existing external-skill warnings plus one External/stitchflow warning because the imported skill includes a workflows/ directory; there were no unique hard validation errors beyond that.

[yshishenya]

Quick update: StitchFlow README and packaging were improved after this PR was opened. The repo now has clearer positioning, polished install flow, richer topics/metadata, and live distribution pages (for example: https://agentskill.sh/@yshishenya/stitchflow). If useful for review, the canonical repo is still https://github.com/yshishenya/stitchflow .

[skeletor-js]

Please rework this into a cleaner external skill submission.

• Remove directories and artifacts that do not belong in the registry package.
• Do not depend on a machine-specific local path like ${HOME}/.agents/stitch-starter.
• Keep the PR scoped to the minimum files needed for a clean external skill.