Create custom-command agents from the web UI (not only Claude skills)

[Beroni]

Description

Let the config dialog create a custom-command agent (name, command, args, free-text task), not only agents built from installed Claude Code skills.

Motivation

The engine already supports any stdin/stdout CLI via per-agent command/args overrides, but the page's add-agent flow only lists ~/.claude/skills and generates /skill {{number}} tasks — which only the claude CLI understands. Today a Codex or Gemini lens requires hand-editing ~/.bazel/config.yaml, while the product's stated rule is "everything is managed from the page".

Proposed Solution

• A second path in the add-agent dialog: custom command — fields for name, description, command, args, task (multiline), and the existing posts toggle.
• Validate that the command resolves in PATH at save time (same spirit as refusing skills that aren't installed — failing only at run time is too late).
• The agent list view shows the command for custom agents where it shows the invoked skill for skill agents.
• POST /api/agents grows the corresponding fields; skill-based creation is unchanged.

Acceptance Criteria

• A codex/gemini agent can be created entirely from the page
• A command not found in PATH is rejected with a clear message at creation
• Custom agents appear in the selector and in pipelines like any other

Related: #11, #13.