fix(skills): auto-detect GitHub URL in skills add command

[christso]

Summary

• Auto-detect when a GitHub URL is passed as the positional skill argument in allagents skills add
• Extract the skill name from the URL (last path segment for URLs with subpath, SKILL.md frontmatter name or repo name for bare repo URLs)
• Treat the URL as the --from plugin source automatically

Closes #256

Changes

• Added resolveSkillFromUrl() helper — pure URL parsing that returns { skill, from, parsed } or null
• Added resolveSkillNameFromRepo() helper — async, fetches repo and reads SKILL.md frontmatter for skill name
• Wired both into the addCmd handler, before findSkillByName()
• Added error for conflicting --from flag when URL is auto-detected
• 9 unit tests covering URL parsing and frontmatter resolution

E2E Test

tmpdir=$(mktemp -d)
cd "$tmpdir"
mkdir -p .allagents
printf 'repositories: []\nplugins: []\nclients: [claude]\n' > .allagents/workspace.yaml
npx allagents skills add https://github.com/anthropics/skills/tree/main/skills/skill-creator

Result:

Skill 'skill-creator' not found. Installing plugin: https://github.com/anthropics/skills/tree/main/skills/skill-creator...
Running initial sync...

Skill 'skill-creator' is already enabled.

Clean up: rm -rf "$tmpdir"

Test plan

• Unit tests pass (9 new tests, 973 total)
• Lint, typecheck, test hooks pass
• E2E: skills add <github-url-with-subpath> installs and enables skill
• E2E: skills add <bare-repo-url> reads name from SKILL.md frontmatter (frontmatter-test-skill extracted from root SKILL.md, not repo name)
• E2E: skills add <url> --from <source> shows conflict error

[cloudflare-workers-and-pages]

Deploying allagents with    Cloudflare Pages

Latest commit:	a387348
Status:	✅  Deploy successful!
Preview URL:	https://e42421a9.allagents.pages.dev
Branch Preview URL:	https://fix-256-skills-add-github-ur.allagents.pages.dev

View logs