ci: add docs sync workflow to keeperhub

[eskp]

Summary

• Add GitHub Actions workflow that syncs CLI command reference to the keeperhub docs-site
• Triggers on release publish or manual dispatch
• Regenerates docs from Go source, copies to keeperhub docs/cli/, creates a PR

How it works

1. Checks out both cli and keeperhub repos
2. Runs go generate ./docs/... to regenerate cobra markdown
3. Copies kh*.md to keeperhub/docs/cli/commands/
4. Adds Nextra frontmatter to quickstart.md and concepts.md
5. Regenerates commands/_meta.ts from filenames
6. Fixes internal links for Nextra routing
7. Creates a PR in keeperhub targeting staging

Setup required

• Add KEEPERHUB_PAT secret: a token with contents:write and pull-requests:write on techops-services/keeperhub

Test plan

• Add KEEPERHUB_PAT secret to repo settings
• Trigger workflow manually to verify end-to-end