docs(agents): make AGENTS.md the only instruction file and require platform-agnostic work

Adds AGENTS.md with the binding rule that skills, scripts, hooks and installs work on every agent runtime (Claude Code, Codex, Gemini CLI, Hermes, OpenClaw) and every OS: logic in scripts, runtime hooks as thin adapters, cross-runtime install locations, no single package manager. Claude Code reads AGENTS.md natively since v2.1.277, so no CLAUDE.md or GEMINI.md exists; kvb-agent-optimize now flags CLAUDE.md, .claude/CLAUDE.md, CLAUDE.local.md and GEMINI.md as drift to fold into AGENTS.md.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

AGENTS.md

@@ -0,0 +1,32 @@
+# Agent Instructions
+
+## Platform-agnostic development
+
+Everything in this repository MUST work on every agent runtime, not only on
+the one developing it: Claude Code, Codex, Gemini CLI, Hermes, OpenClaw and
+any other that reads skills. The same holds for operating systems and
+distributions.
+
+- **Logic in scripts**: behaviour lives in `python3` or `bash` scripts that
+  any runtime can call. Hooks, plugins and settings of one runtime are thin
+  adapters over those scripts, never the only path to a feature.
+- **Skills stay portable**: a `SKILL.md` that uses a runtime-specific tool
+  (`AskUserQuestion`, `SendMessage`, a slash command) says what the step
+  needs, so another runtime can use its own equivalent.
+- **Installs reach every runtime**: install into the cross-runtime locations
+  (`~/.agents/skills`) or through an installer that configures every
+  detected agent; runtime-specific locations are mirrors. Of a tool that
+  ships both, take the agent-agnostic integration over one runtime's plugin,
+  and never both: the same server would run twice.
+- **No single package manager**: an install never depends on one OS or
+  distribution's package manager; prefer upstream or statically linked
+  release builds.
+- **One instruction file**: rules live here and nowhere else; no
+  `CLAUDE.md`, `CLAUDE.local.md` or `GEMINI.md`. Claude Code reads this file
+  natively from v2.1.277; Gemini CLI once its `context.fileName` setting
+  lists `AGENTS.md`.
+
+## Code questions
+
+Ask the codebase-memory-mcp graph first (`search_graph`, `trace_path`,
+`get_architecture`); grep only after it.