feat(agent-settings): add enable/disable toggle to detected ACP agents

[szafranski]

Description

Detected ACP agent cards in Settings → Agent Management were read-only: they
only had a "Go to chat" button, while custom agents already had an enable/disable
Switch. This adds the same on/off toggle to detected ACP agents, so users can
turn off vendors they don't want cluttering the agent picker. Toggling off greys the
card, disables its "Go to chat", and removes the agent from the chat picker (the
shared cache is revalidated); toggling on reverses all of it.

The built-in Aion CLI card intentionally keeps no switch and stays always-on.
A disabled Internal/no-command agent fails the backend's is_disabled_but_installed
check (probe_command returns NoCommand), so it would not re-surface in the
management view and would vanish with no way to re-enable it from the GUI. Omitting
the switch on that one card avoids the trapdoor.

This is a pure renderer change. No backend change is needed: PATCH /api/agents/:id/enabled already operates generically on any row by id, and the
management view already lists disabled-but-installed agents (via useManagedAgents).

Related Issues

Type of Change

• New feature (non-breaking change which adds functionality)

Testing

• Tested on macOS
• Tested on Windows
• Tested on Linux
• My code follows the project's code style guidelines
• I have performed a self-review of my own code
• My changes generate no new warnings or errors

Local checks: oxfmt, oxlint (0 errors), tsc --noEmit, i18n validation, full
vitest suite (1402 passed / 3 skipped). New AgentCard detected-variant tests cover
switch present/absent and enabled/disabled branches.

Additional Context

The Aion CLI scope decision is deliberate (see Description). No new i18n strings: the
switch has no label and "Go to chat"/"Detected" reuse existing keys.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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