Add the documentation and the agent instructions

README for humans, AGENTS.md for agents and contributors, and the docs that
outlive any one platform: architecture, secrets, connectivity, triggering a
deploy from another repository. CLAUDE.md, GEMINI.md and .claude/skills/ are
pointers rather than copies, so every agent and every human reads the same
text.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

GEMINI.md

@@ -0,0 +1,6 @@
+# GEMINI.md
+
+The instructions for this repository live in `AGENTS.md`, so that every agent and
+editor reads the same file. Do not duplicate guidance here — edit `AGENTS.md`.
+
+@AGENTS.md