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.

.claude/CONTEXT.md

@@ -24,6 +24,12 @@ _Avoid_: loader, importer, scanner
 The rule that every Module is imported unconditionally and guards its own body with `mkIf config.modules.<path>.enable`, so a Host reads as a checklist of `enable = true` flags.
 _Avoid_: feature flag, toggle, opt-in
 
+**Namespace convention**:
+The rule that a Module's option path mirrors its directory path under `modules/`, so a file's location is its namespace.
+A file whose name matches its enclosing directory is that directory's index node, declaring the directory's own segment rather than a doubled one.
+A directory with no such file is a pure namespace prefix that carries no aggregate enable.
+_Avoid_: option tree, module path, config key
+
 **admin identity**:
 The age identity held only in the operator's password manager, never committed, that is a recipient of every secrets file.
 It is the recovery path for any wiped machine and the credential that authorizes registering a new host.