feat(da-auth): add da-auth skill and surface DA auth in CDD workflow

[kptdobe]

Summary

• Adds a new da-auth sub-skill for obtaining a valid Adobe IMS token before making DA Admin API calls
• Updates content-driven-development (Step 4 DA callout + Related Skills)
• Updates building-blocks (Related Skills)

Motivation

During a real AEM EDS project session, an agent had to be manually guided through DA authentication every time it needed to push content or trigger a preview. The da-auth-helper tool and DA Admin API were only referenced in create-site (new-site onboarding), leaving agents in any other context — content updates, block development, test content creation — with no guidance.

What the da-auth skill does

1. Step 1 — Check token cache at ~/.aem/da-token.json (skip login if still valid)
2. Step 2 — Three fallback options: da-auth-helper CLI (preferred), DA MCP server, manual paste
3. Step 3 — Verify the token works against admin.da.live before returning

The description frontmatter tells the agent not to re-invoke if a token is already in scope, preventing unnecessary re-auth mid-session.

Changes

plugins/aem/edge-delivery-services/skills/
├── da-auth/                          ← NEW
│   ├── SKILL.md
│   ├── package.json
│   └── .releaserc.json
├── content-driven-development/
│   └── SKILL.md                      ← Step 4 DA callout + da-auth in Related Skills
└── building-blocks/
    └── SKILL.md                      ← da-auth in Related Skills

What was NOT changed

• create-site — already handles DA auth well as its own Step 4; no change needed

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