feat(agents): split global agent config into shared base + per-agent wrappers

- agents/AGENTS.md: shared, agent-agnostic personal preferences
- agents/AGENTS-claude.md: Claude Code wrapper (@AGENTS.md + @RTK.md)
- agents/AGENTS-opencode.md: OpenCode wrapper (@AGENTS.md)
- Old claude/CLAUDE.md removed; symlinks updated in ~/.claude and ~/.config/opencode

This lets each agent layer in tool-specific content (e.g. the Agent tool
tiering for Claude Code, RTK hooks) without leaking it into other agents.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

README.md

@@ -6,7 +6,9 @@ Shared shell and tool configuration.
 
 Tracked files:
 - `.zshrc` — shared zsh UI behavior
-- `claude/CLAUDE.md` — global Claude Code instructions
+- `agents/AGENTS.md` — shared global personal preferences (agent-agnostic)
+- `agents/AGENTS-claude.md` — Claude Code wrapper (`@AGENTS.md` + `@~/.claude/RTK.md`)
+- `agents/AGENTS-opencode.md` — OpenCode wrapper (`@AGENTS.md`)
 
 This repo is intentionally small. Machine-specific setup stays local unless explicitly promoted into shared config later.
 
@@ -18,12 +20,15 @@ This repo is intentionally small. Machine-specific setup stays local unless expl
 source "$HOME/Projekte/dotfiles/.zshrc"
 ```
 
-**Claude Code**: `~/.claude/CLAUDE.md` is a symlink to this repo:
+**Claude Code & OpenCode**: symlink the wrappers into each agent's config dir:
 
 ```zsh
-ln -s "$HOME/Projekte/dotfiles/claude/CLAUDE.md" ~/.claude/CLAUDE.md
+ln -s "$HOME/Projekte/dotfiles/agents/AGENTS-claude.md"   ~/.claude/CLAUDE.md
+ln -s "$HOME/Projekte/dotfiles/agents/AGENTS-opencode.md" ~/.config/opencode/AGENTS.md
 ```
 
+The wrappers `@`-include the shared base `agents/AGENTS.md`; add agent-specific content directly in the wrapper file.
+
 ## What Lives Here
 
 Current shared config includes:

agents/AGENTS-claude.md

@@ -0,0 +1,3 @@
+@./AGENTS.md
+
+@~/.claude/RTK.md

agents/AGENTS-opencode.md

@@ -0,0 +1 @@
+@./AGENTS.md

agents/AGENTS.md

@@ -25,5 +25,3 @@
   it, results are filtered to the current working directory's
   subtree. The `ccc` skill has the full reference;
   `grep`/`rg`/`find` remain fine for exact-string lookups.
-
-@RTK.md