v0.2.8 — Antigravity adapter + paste-to-install

First-class Google Antigravity support + a paste-to-install path for non-terminal users.

Antigravity registration & routing

• /hephaestus ships as an Antigravity workflow: antigravity/workflows/hephaestus.md (canonical) + .agents/workflows/hephaestus.md (project scope).
• scripts/install-all-runtimes.sh installs the global workflow to ~/.gemini/antigravity/global_workflows/hephaestus.md → /hephaestus in every Antigravity workspace.
• Antigravity already auto-loads AGENTS.md and .agents/skills/; this adds the missing command surface. Registry/manifest/AGENTS.md/GEMINI.md updated; antigravity is a verified runtime.

Paste-to-install (README ×5)

• New top-of-README block: one natural-language message you paste into any AI tool (Claude Code · Codex · Gemini CLI · Antigravity · Cursor) so the agent runs the installer for you.

Release

• Version bump 0.2.7 → 0.2.8. All verify scripts pass (contract, install-docs, package incl. 12 ontology tests, public-safety).

Merged via #1.