feat: add rhdh-coding skill for Backstage/RHDH plugin development

[rohitkrai03]

Why

RHDH is moving toward an agentic SDLC with fullsend — agents triage issues, implement features, and review code autonomously. The coding agent needs Backstage/RHDH patterns that aren't in training data (BUI is new, NFS is evolving, dynamic plugin conventions are tribal knowledge). No Backstage coding skills exist in the open-source ecosystem — the official backstage.io skills cover migrations and instrumentation, not day-to-day feature development.

This skill fills that gap. It's the daily-driver skill for agents working on RHDH plugins — installed into .fullsend/customized/skills/ alongside rhdh-workspace and available to the code agent during implementation. It also helps human developers using Claude Code, Cursor, or any skills-compatible agent.

What it covers

• BUI-first styling — component mapping, CSS variables, Remix icons (too new for training data)
• NFS Blueprints — PageBlueprint, EntityCardBlueprint, alpha exports, NFS-at-root pattern
• RHDH dynamic plugins — default export gotcha, scalprum naming, class name generator
• Dev app setup — both plugin dev mode and full Backstage app with sidebar nav
• Backstage-specific testing — TestApiProvider, renderInTestApp, entity mocking, MSW
• Plugin type decisions — page vs card vs entity tab vs backend module
• Frontend spec guidance — what to include in PRDs/OpenSpec proposals for frontend features
• Version compatibility — RHDH → Backstage version matrix awareness

How it fits into fullsend

# .fullsend/customized/harness/code.yaml
skills:
  - skills/code-implementation    # base fullsend
  - skills/rhdh-workspace         # monorepo navigation
  - skills/rhdh-coding            # Backstage/RHDH patterns (this skill)

Complements ecosystem skills (ui-ux-pro-max, frontend-design, webapp-testing) for generic UI quality, and official backstage.io skills for migration workflows.

Summary

• 9 files, ~1,200 lines
• References verified against real plugin code in rhdh-plugins and community-plugins
• Tested against a real NFS plugin implementation (boost workspace) — feedback incorporated
• Detection script verified against orchestrator and bulk-import plugins

Files

File	Lines	Purpose
SKILL.md	176	Core skill — principles, BUI-first, NFS, RHDH gotchas, sibling + official skill references
references/bui.md	149	BUI component mapping, CSS variables, Remix icons
references/dev-app.md	212	Plugin dev mode + full Backstage app setup
references/nfs.md	155	NFS Blueprints, package exports, compatWrapper
references/frontend-specs.md	86	Frontend spec guidance for PRDs/OpenSpec
references/plugin-types.md	79	Plugin type decision guide
references/rhdh.md	141	RHDH-specific patterns
references/testing.md	122	Backstage test patterns
scripts/detect-rhdh-context.py	163	Plugin context detection

Test plan

• Detection script tested against orchestrator and bulk-import plugins
• Skill tested in Cursor against real NFS plugin implementation (boost workspace)
• Real usage feedback incorporated (dev-app gaps, NavItemBlueprint removal, PageBlueprint title, translation auto-discovery, NFS-at-root pattern)
• Install via npx skills add and verify skill triggers correctly

🤖 Generated with Claude Code

[durandom]

Review

Comprehensive skill capturing Backstage/RHDH plugin development tribal knowledge. This is the kind of content that saves developers hours of debugging.

What's good

• BUI reference (`bui.md`) — the component mapping table (BUI ↔ MUI) with key API differences is exactly what's needed for migration. Calling out `Card`'s discriminated union, `Select`'s onChange signature, and `Table`'s `isRowHeader` requirement prevents real bugs.
• NFS patterns (`nfs.md`) — alpha file structure, PageBlueprint/EntityContentBlueprint patterns, and the package exports section (NFS-at-root vs legacy-at-root vs alpha pattern) addresses a genuine source of confusion.
• Testing reference (`testing.md`) — TestApiProvider patterns, entity context mocking approaches (EntityProvider vs jest.mock), MSW setup, and the SWR cache reset trick are all verified patterns from real plugins.
• Detection script — stdlib-only Python, clean structure, covers all the right signals (backstage role, frontend system, MUI version, extensions, blueprints, route refs, API refs, dynamic plugin status).
• "Before You Commit" checklist — the ordered sequence (`prettier:fix` → `tsc:full` → `build:all` → `test` → `build:api-reports:only`) with "stop on first failure" is practical.

Issues

1. Merge conflicts — this PR has conflicts with main and no CI runs. Needs a rebase before it can be reviewed for merge readiness.

2. Version bump assumes #49 merged. The diff shows 0.6.0 → 0.7.0, meaning this is based on PR #49's branch. If #49 hasn't merged, this can't either. Clarify the dependency chain.

3. `references/bui.md` caveat. The skill says "Always check `node_modules/@backstage/ui/dist/index.d.ts` for actual prop types" — good. But the BUI component list will go stale as BUI evolves. Consider adding a date or BUI version at the top so readers know when it was last verified.

4. `references/nfs.md` — NavItemBlueprint removal note. "NavItemBlueprint was removed" — this is correct for latest Backstage but worth noting which version this changed in, since plugins targeting older RHDH versions may still need it.

5. Ecosystem skills section. Recommending `npx skills add` for third-party skills is useful, but these URLs/repos could go stale. Consider moving this to a separate reference file that's easier to update.

Recommendation

Excellent content. Blocked on rebase (conflicts) and the #49 dependency. Once those are resolved, this is merge-ready.