feat(cli): add zcode agent profile

[twodogegg]

Summary

• Adds a first-class zcode agent profile (display name ZCode) so Skills can be installed with --agent zcode:
  • project-level: <project>/.zcode/skills/
  • user-level: ~/.zcode/skills/
• Follows the existing makeProfile pattern with automatic detection of both scopes, and registers the profile in allProfiles / profileMap.
• Updates the install-path tables in cli/README.md and docs/skillhub/{,en/}guide/cli.md, adds a cli/CHANGELOG.md entry, and extends test/unit/agents/profiles.test.ts for the new profile.

Validation

• Backend tests passed (bun test: 561 pass, 0 fail)
• Frontend typecheck/build passed (bun run typecheck, bun run build)
• OpenAPI SDK regenerated or checked when API contracts changed (no API contract changes)
• Smoke test run when relevant (built CLI resolves --agent zcode to ~/.zcode/skills and detects an existing installation there)

Commands run:

cd cli
bun install
bun run typecheck
bun test
bun run build
node dist/index.js list --agent zcode --registry <registry> --json
node dist/index.js install @global/skillhub-cli --version 2.0.2 --scope user --agent zcode --registry <registry> --json

Risk

• User-facing impact: New --agent zcode choice and auto-detection of .zcode/skills directories; no change to existing agents or default behavior.
• Deployment or migration impact: None.
• Rollback approach: Revert the commit.

Notes

• Related issue: none
• Follow-up work: none
• Docs or operator runbooks updated when behavior changed: yes (cli/README.md, docs/skillhub/guide/cli.md, docs/skillhub/en/guide/cli.md, cli/CHANGELOG.md)

[CLAassistant]

All committers have signed the CLA.