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/tasks/0031-module-namespace-tidy.md

@@ -0,0 +1,29 @@
+## What to build
+
+Tidy the module tree so a Module's option namespace mirrors its directory under `modules/`, adopt that as a documented convention, group the agent-related Modules under a new `agents/` directory, bring `desktop/hyprland/` into conformance, and drop the obsolete reference Module.
+
+The convention: a Module's option path mirrors its directory path, and a file whose name matches its directory is that directory's index node — it declares the directory's own segment (its `enable`/aggregator) rather than a doubled segment. A file `foo.nix` in directory `d/` declares `modules.<…>.d.foo`. A group directory with no matching index file contributes a namespace segment but no aggregate `enable`.
+
+Applying it:
+
+- **Agents grouping.** Relocate the agent Modules under `modules.agents.*`: `claude-code` (its whole directory, assets included) → `modules.agents.claude-code`; `pi` flattened from its directory to a single file → `modules.agents.pi`; the skills Module renamed from `agent-skills` → `agents/skills.nix`; and `gitea-axi` into an `agents/tools/` subgroup → `modules.agents.tools.gitea-axi`. `tools/` is a real namespace segment, not a cosmetic folder.
+- **No aggregators.** `agents/` and `tools/` are pure namespace prefixes — no `modules.agents.enable` or `modules.agents.tools.enable`. Agents are enabled à la carte.
+- **Skills stays enable-less.** The skills Module keeps its current behaviour (unconditionally wires `programs.agents.skills`, empty list); it is the one deliberate exception to the Enable convention, marked as intentional by a self-contained comment in the file.
+- **Desktop conformance.** Nest `hypridle` and `hyprlock` under `modules.desktop.hyprland.*` (matching the index-file rule, `hyprland.nix` being the index), and update `desktop.nix`'s aggregator to the new paths. The 13 flat `desktop/*.nix` Modules keep their `modules.desktop.<name>` names — broader semantic regrouping is explicitly out of scope for this task.
+- **Remove the example Module.** Delete `modules/example.nix`; the documented convention and the many real Modules supersede its teaching role.
+
+Also update the one Host that carries these Modules and the live documentation, and record the convention in the domain model.
+
+## Acceptance criteria
+
+- [x] `CONTEXT.md` gains a `Namespace convention` glossary entry stating the directory-mirrors-namespace rule and the index-file rule, in glossary style (no implementation detail).
+- [x] An ADR (next number: `0004`) records the decision — nested-mirrors-directory over flat names, a subfolder as a real namespace segment, the index-file rule, and no `agents` aggregator — following the ADR format.
+- [x] Agent Modules resolve under `modules.agents.*`: `modules.agents.claude-code.enable`, `modules.agents.pi.enable`, and `modules.agents.tools.gitea-axi.enable` exist; `modules.claude-code`, `modules.pi`, and `modules.gitea-axi` no longer resolve.
+- [x] The skills Module lives at `agents/skills.nix` (renamed from `agent-skills.nix`), stays enable-less, still wires `programs.agents.skills`, and carries an in-file comment marking the Enable-convention exception as intentional.
+- [x] Neither `modules.agents.enable` nor `modules.agents.tools.enable` exists (pure namespace prefixes, no aggregator).
+- [x] `claude-code`'s assets (`CLAUDE.md`, `authentication.md`, `hooks/`, `skills/`) travel with the move and its relative references still resolve.
+- [x] `desktop/hyprland/`: `modules.desktop.hyprland.hypridle` and `modules.desktop.hyprland.hyprlock` resolve; the old `modules.desktop.hypridle`/`modules.desktop.hyprlock` no longer exist; `desktop.nix` enables the new paths; `modules.desktop.enable` still brings up the whole session.
+- [x] `modules/example.nix` is removed and `modules.example` no longer resolves.
+- [x] `hosts/neogaia/default.nix` uses the new option paths for claude-code, pi, and gitea-axi.
+- [x] The two live `CLAUDE.md` gotchas — the `gitea-axi` install line and the `claude-code` skill-source path — are updated to the new option/path; `.claude/tasks/*` are left unchanged as historical record.
+- [x] `nix flake check` builds `checks.x86_64-linux.neogaia` green (moved files staged so evaluation sees them).