agents: extract deep-dive content into on-demand skills
Move detailed runtime internals documentation (experimental flags,
pre-compiled bundles, DCE patterns, React vendoring) from always-loaded
AGENTS.md into focused skills under .agents/skills/. AGENTS.md keeps
one-liner guardrails and points to skills for deep-dive workflows.
Skills added:
- flags: feature-flag wiring end-to-end
- dce-edge: DCE-safe require patterns and edge constraints
- react-vendoring: entry-base boundaries and vendored React
- runtime-debug: bundle regression diagnosis workflow
- pr-status-triage: CI failure and PR review triage
- authoring-skills: how to create and maintain skills
AGENTS.md: 489 -> 426 lines (-13%), deep content now loads on demand.
