add canonical agent instructions with per-tool symlinks

Store agent instructions at ~/.config/agents/AGENTS.md and symlink
~/.copilot/copilot-instructions.md and ~/.config/opencode/AGENTS.md to it.
Instructs agents to omit attribution trailers and to get the commit message
approved before committing.

dot_config/agents/AGENTS.md

@@ -0,0 +1,6 @@
+# Agent instructions
+
+## Commits
+
+- Do not add a `Co-authored-by` trailer (or any other agent attribution) to commit messages.
+- Before committing, show the proposed commit message and wait for approval. Apply any edits I request, then commit.

dot_config/opencode/symlink_AGENTS.md.tmpl

@@ -0,0 +1 @@
+{{ .chezmoi.homeDir }}/.config/agents/AGENTS.md

dot_copilot/symlink_copilot-instructions.md.tmpl

@@ -0,0 +1 @@
+{{ .chezmoi.homeDir }}/.config/agents/AGENTS.md