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/tools/gitea-axi.nix

@@ -0,0 +1,22 @@
+{
+  config,
+  lib,
+  inputs,
+  ...
+}:
+# gitea-axi for the primary user, installed through its own home-manager module.
+# That module also declares the Claude Code context when that harness is
+# enabled on the host; enabling this alone installs the CLI and nothing else.
+let
+  cfg = config.modules.gitea-axi;
+  user = config.user.name;
+in
+{
+  options.modules.gitea-axi.enable =
+    lib.mkEnableOption "gitea-axi, an agent-ergonomic CLI for Gitea issues and pull requests";
+
+  config = lib.mkIf cfg.enable {
+    home-manager.sharedModules = [ inputs.gitea-axi.homeModules.default ];
+    home-manager.users.${user}.programs.gitea-axi.enable = true;
+  };
+}