feat: support installing skills from specific git branches and keep updates pinned

[boyfeiyu]

Background

openskills install currently clones the default branch only.
This makes it hard to test/use skills from develop, release/*, or tagged branches.
Also, openskills update always re-clones default branch, so a skill originally installed from a non-default branch cannot stay pinned to that branch.

What this PR changes

1. Add branch selection for install:

• New flag: -b, --branch <name>
• Inline syntax: <source>#<branch>

2. Keep branch information in metadata:

• Persist branch in .openskills.json for git-based installs

3. Make update branch-aware:

• openskills update now uses recorded branch (if present) when cloning

4. Input validation:

• Reject empty inline branch (source#)
• Reject duplicate branch input (--branch + source#branch)
• Reject --branch for local path installs (git-only option)

5. Docs:

• Added branch/tag install examples and the new CLI flag in README

6. Test reliability improvement:

• e2e tests now auto-run npm run build if dist/cli.js is missing, so npm test works in a clean environment.

User-facing examples

npx openskills install your-org/your-skills --branch develop
npx openskills install your-org/your-skills#develop