refactor: mirror module namespaces to their directories (task 0031)

Adopt the convention that a Module's option path mirrors its directory
under modules/, with an index file naming the directory's own segment.

- Group agent Modules under modules.agents.*: claude-code (whole
  directory), pi (flattened to a file), skills (renamed from
  agent-skills), and gitea-axi under an agents/tools/ subgroup. The
  agents/ and tools/ folders are pure namespace prefixes with no
  aggregator enable.
- Nest hypridle and hyprlock under modules.desktop.hyprland.*, with
  hyprland.nix as the index, and update the desktop aggregator.
- Remove the obsolete example Module.
- Record the convention in CONTEXT.md and ADR 0004, and update the
  neogaia host, the two live CLAUDE.md gotchas, and the skills Module's
  intentional Enable-convention exception comment.

modules/agents/pi.nix

@@ -0,0 +1,29 @@
+{
+  config,
+  lib,
+  ...
+}:
+# Pi, a terminal coding agent, for the primary user, configured through
+# home-manager, which ships the package and manages ~/.pi/agent.
+# The login credential is left unmanaged, so it survives rebuilds.
+let
+  cfg = config.modules.pi;
+  user = config.user.name;
+in
+{
+  options.modules.pi.enable = lib.mkEnableOption ''
+    Pi, a terminal coding agent, configured via home-manager'';
+
+  config = lib.mkIf cfg.enable {
+    home-manager.users.${user}.programs.pi-coding-agent = {
+      enable = true;
+
+      settings = {
+        defaultProvider = "anthropic";
+        # Pi's catalogue id for Opus.
+        defaultModel = "claude-opus-4-8";
+        enableAnalytics = false;
+      };
+    };
+  };
+}