fix(agents): add include_disabled query to GET /api/agents

[szafranski]

What

Add an opt-in include_disabled query parameter to GET /api/agents.

By default the endpoint is unchanged: it returns only enabled && available rows, so
pickers keep showing only spawnable agents. With ?include_disabled=true the response
additionally includes rows hidden solely because the user disabled them
(enabled = false) whose spawn command still resolves on $PATH. Rows hidden because the
binary is missing stay hidden in both modes.

Why

Disabling a custom agent currently removes it from every UI surface, including the AionUi
settings screen that hosts the re-enable toggle — leaving no way to turn it back on from the
GUI. The root cause is backend-side: GET /api/agents filters out every non-visible row.
This adds the management view the settings screen needs while keeping the picker contract
intact.

See the AionUi issue for the end-to-end report: iOfficeAI/AionUi#3318

How

• aionui-api-types: new ListAgentsQuery { include_disabled: bool }.
• routes/agent.rs: list_agents now extracts Query<ListAgentsQuery> and threads the flag.
• services/agent.rs: list_agents(include_disabled) forwards to the registry.
• registry.rs:
  • new list_for_view(include_disabled) whose predicate is
    is_visible(m) || (include_disabled && is_disabled_but_installed(m));
  • probe_resolved_command is refactored to split the disabled guard out into a new
    probe_command helper, so a disabled-but-installed row can be told apart from a
    disabled-and-missing one without changing the available field. Disabled rows keep
    available = false; the renderer greys them off enabled, not available.
• refresh_agents (POST /api/agents/refresh) intentionally stays on the default filtered
  view — the settings screen drives the list endpoint, not refresh, so there is no need to
  surface disabled rows there.

Tests

cargo test -p aionui-ai-agent — added registry tests:

• a disabled-but-installed custom row is absent from the default view, present with
  include_disabled=true;
• a CLI-missing row stays absent in both views (no over-relaxing);
• an enabled+installed row is present in both views.

Coupling

This is the backend half of a cross-repo fix. The renderer half is a separate PR in
iOfficeAI/AionUi (which uses the new parameter from the settings screen only). Both PRs must
be bundled together for the fix to function in a release.

[szafranski]

Small release-coordination bump: AionUi v2.1.19 already includes the renderer half in iOfficeAI/AionUi#3319 (fix(agents): keep disabled custom agents visible in settings), but the bundled AionCore v0.1.30 does not include this backend half yet.

That means users on the latest release can still see the old behavior, because the settings UI now calls /api/agents?include_disabled=true and needs this endpoint support to make the promised fix actually work. Could we please make sure this lands and is bundled in the next AionCore/AionUi release? Thanks!