feat: config with agents rework

AGENTS.md

@@ -106,6 +106,37 @@ meta = with lib; {
 
 **Shared lib**: `portsLib = import ../../lib/ports.nix { inherit lib; }; portHelpers = portsLib.mkPortHelpers { ... };`
 
+## LIBRARY FUNCTIONS
+
+### `lib.ports`
+
+Port management utilities. See [Port Management](#port-management).
+
+### `lib.agents`
+
+Harness-agnostic agent management. Reads canonical `agent.toml` from the AGENTS
+flake input and renders tool-specific configs.
+
+**Functions:**
+
+| Function | Purpose |
+|----------|--------|
+| `loadCanonical { agentsInput }` | Load canonical agents from AGENTS flake |
+| `renderForOpencode { pkgs, canonical, modelOverrides }` | Render to OpenCode file-based agents |
+| `renderForClaudeCode { pkgs, canonical, modelOverrides }` | Render to Claude Code agents + settings.json |
+| `renderForPi { pkgs, canonical }` | Render to Pi AGENTS.md + SYSTEM.md |
+| `renderForTool { pkgs, agentsInput, tool, modelOverrides }` | Dispatch to correct renderer |
+| `shellHookForTool { pkgs, agentsInput, tool, modelOverrides }` | Generate devShell shellHook |
+
+### `lib.coding-rules`
+
+Coding rules injection (renamed from `lib.opencode-rules`). The old name still works.
+
+| Function | Purpose |
+|----------|--------|
+| `mkCodingRules { agents, languages, concerns, frameworks }` | Generate rules config + shellHook |
+| `mkOpencodeRules` | Backward-compat alias for `mkCodingRules` |
+
 ## PORT MANAGEMENT
 
 Central port management: `config.m3ta.ports.get "service"` with host-specific via `hostOverrides`
@@ -168,3 +199,27 @@ Use `td usage -q` for subsequent reads.
 - `td version` - Check version
 
 For full workflow details, see the [td documentation](./docs/packages/td.md).
+
+## MIGRATION: Agent System (OpenCode → Canonical TOML)
+
+The agent system was migrated from embedded `agents.json` to harness-agnostic
+canonical `agent.toml` + `system-prompt.md` in the AGENTS repo. Renderers in
+`lib/agents.nix` generate tool-specific configs.
+
+### What changed in this repo
+
+- **`lib/agents.nix`**: New — 3 renderers (OpenCode, Claude Code, Pi) + dispatcher + shellHook
+- **`lib/coding-rules.nix`**: Renamed from `opencode-rules.nix`, `mkCodingRules` replaces `mkOpencodeRules`
+- **`modules/home-manager/coding/agents/`**: New — per-tool HM sub-modules
+- **`modules/home-manager/coding/opencode.nix`**: Slimmed — no longer handles agents/skills/context
+- **`flake.nix`**: Exports new `agents` HM module
+
+### What the user must do
+
+See `modules/home-manager/AGENTS.md` for the full migration guide. Summary:
+
+1. Move `agentsInput`/`externalSkills` from `coding.opencode` to `coding.agents.opencode`
+2. Add `modelOverrides` with previously hardcoded model strings
+3. Run `home-manager switch`
+4. Remove legacy `agents.json` + `prompts/*.txt` from AGENTS repo
+5. Remove `lib.agentsJson` backward-compat bridge from AGENTS `flake.nix`