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>

.claude/skills/adding-a-stack/SKILL.md

@@ -0,0 +1,10 @@
+---
+name: adding-a-stack
+description: Use when adding a new service to this repository, moving a service to a different host, or bringing a hand-deployed service under Ansible and CI.
+---
+
+# Adding a stack
+
+The instructions live in [`skills/adding-a-stack/SKILL.md`](../../../skills/adding-a-stack/SKILL.md),
+outside `.claude/` so that every agent reads the same file. Read it and follow
+it. Do not duplicate guidance here.