(agents): rework the New background agent dialog: overlapping rows, misaligned Project select, CLI jargon, modes #501

The New background agent dialog of the Agents view (#374, public/dialogs.js) has layout and content defects, seen on the v0.0.90 candidate (PR #500, test instance at 1400x900).

Layout

• The Prompt textarea runs under its row separator and touches the Project label.
• The Project select is narrower than the other controls and not aligned with them: its right edge and width differ from the Name and Agent fields.
• The Permission Mode grid overlaps the description of the Agent row (the "Default" card crosses its text), and the "Dangerous Skip" card and the Additional Directories field run under their row separators.
• The "Permission Mode" label is vertically centred against a tall grid instead of aligned to the top.

Rows seem to keep the fixed height of the Settings rows they reuse; taller controls overflow.

Content

• Descriptions show CLI flags (--name, --agent) instead of what the field does.
• All seven permission modes are offered. A background agent has nobody to answer a prompt: "Default" and "Don't Ask" stall or refuse silently, and "Bypass" / "Dangerous Skip" read as duplicates. Proposed set: Accept Edits (default), Auto, Plan, Bypass.
• Agent is free text although the available agents are known; a select avoids typos (keep "none").
• Additional Directories splits on commas, which breaks on a path containing one; one path per line instead.

Acceptance

• An e2e journey opens the dialog at 1400x900 and at a narrow window, and asserts that no control's box overlaps another row's label, description or separator, and that the Project select, the Name and Agent controls share the same left and right edges.
• Unit tests pin the offered permission modes and their default, the agent list as a select, and the directory parsing.