Revert the AGENTS.md primer; keep the full skill + gallery flow and cache it instead #1613

The AGENTS.md primer (#1592, #1605, #1612) cut build tokens by telling agents to skip the gallery and most of the skill. The owner prefers keeping all three context layers (gallery source, skill, node_modules source) at full strength and making them cheap through a cached context pack loaded before the user's message.

Scope:

• Revert feat: ship AGENTS.md as a one-file WebJs primer #1592, feat: compact primer with UI patterns, upload pattern and a reference table #1605 and fix: primer pages keep a strong focus ring, a saving state and stat tiles #1612 so the scaffold AGENTS.md is again: read the skill, study the gallery, then clear it.
• Keep non-primer fixes: perf: build a production-only image from the scaffold Dockerfile #1607, perf: run a Bun app's db scripts under Bun, not Node #1599, fix: never reload the dev page for files nothing serves #1610, fix: keep the request visible to app code after a Bun dev reload #1591, feat: first-class app icon and manifest files, placeholder scaffold icon #1588, and the SKILL.md cheat-sheet parity fix that rode in feat: ship AGENTS.md as a one-file WebJs primer #1592.
• Follow-up in the same effort: a generated context pack (real gallery files + SKILL.md + core references) imported from the scaffold CLAUDE.md so it sits in Claude Code's cached prefix.