Spawn agent form and POST handler

Create the "Spawn Agent" form page and POST handler.

Spawn Form (GET /agents/new)

• Prompt: Large textarea for the initial prompt/task
• Name: Optional text input for a human-readable run name
• Task: Optional dropdown/text input to associate with a task ID (typeahead from task DB)
• Model: Dropdown with available models (default: claude-sonnet-4-5). Pull list from Omni.Agent.Models.
• Workspace:
• Default: auto-create in daemon's workspace root
• Option: specify existing path (e.g., repo root for worktree creation)
• Checkbox: "Create git worktree" (default on when workspace is a git repo)
• Submit: "Launch Agent" button

POST Handler (POST /agents/spawn)

• Validate form inputs
• Call Agentd daemon POST /agents with SpawnRequest JSON:

json { "prompt": "...", "name": "...", "workspace": "/path/to/repo", "model": "claude-sonnet-4-5-20250514" }

• On success: redirect to /agents/:runId (the new agent's detail page)
• On error: re-render form with error message

HTMX

• Form submission via htmx: hx-post="/agents/spawn" with redirect on success
• Or: standard form POST with 303 redirect

Mobile

• Form should be usable on phone (large touch targets, appropriate input types)
• Textarea should be tall enough for multi-line prompts