fix: support documented generic setup hosts

[sahaavi]

Summary

This finishes setup support for the documented generic hosts that already exist in the host registry but were still rejected by ./setup.

Before this change:

• README.md documented ./setup --host opencode, ./setup --host cursor, and ./setup --host slate
• setup still rejected those values at argument parsing time
• the host registry already contained hosts/opencode.ts, hosts/cursor.ts, and hosts/slate.ts

Root cause

setup still had a hardcoded host allowlist and install flow, while host registration had already moved into the declarative host config system.

What changed

• added a generic setup path driven by host config export data
• accepted config-declared generic hosts in setup --host ...
• added auto-detection for generic setup hosts
• made generic host skill generation refresh on setup runs instead of only when the output directory is missing
• made generic-host eligibility explicit via install.setupStrategy
• fixed OpenCode/Cursor/Slate runtime asset exports so the generated skills and installed runtime root agree:
  • design/dist
  • review/design-checklist.md
  • review/greptile-triage.md
  • review/specialists/*.md
• added the missing .claude/skills/review -> .host/skills/gstack/review path rewrite for OpenCode/Cursor/Slate
• tightened docs so they describe the generic installer accurately
• added tests that assert generic hosts export the runtime assets their generated skills actually reference

Verification

Passed:

• bun test test/host-config.test.ts --test-name-pattern 'hosts/index.ts|validateHostConfig|validateAllConfigs|HOST_PATHS derivation from configs|host-config-export.ts CLI|host config correctness'
• bun test test/gen-skill-docs.test.ts --test-name-pattern 'setup script validation|Parameterized host smoke tests|--host all'
• bun run gen:skill-docs --host all --dry-run
• bun run skill:check

Manual smoke:

• ./setup --host opencode -q
• this no longer fails on Unknown --host value: opencode
• it now proceeds into the existing setup flow and stops later in the pre-existing browser/bootstrap path with:
  • error: cannot write multiple output files without an output directory
  • gstack setup failed: Playwright Chromium could not be launched

Notes

I also confirmed that stale golden baselines in the current checkout are unrelated to this change, so I kept this PR focused on the setup/host-config gap only.