Add per-app agent support with tailored assistants for blog, places, and weather

[Copilot]

Investigates and implements the "agent per app" concept: each app gets a tailored AI agent with its own system prompt and a restricted tool set, surfaced as an inline assistant widget within the app's UI.

Core infrastructure (agent/agent.go)

• AppAgent struct — ID, Name, SystemPrompt, Tools (empty = unrestricted)
• RegisterAppAgent() — package-level registry; any package can register its own agent
• handleQuery — accepts optional "app" field in the POST body; restricts the planner's tool catalogue and swaps in the app-specific synthesis prompt when set
• buildRestrictedToolsDesc() — generates a tool description string scoped to an app's allowed tools
• Load() registers three built-in agents:

ID	Role	Tools
blog	Writing assistant: grammar, clarity, tags, moderation	blog_list, search, web_search
places	Discovery & recommendations: intent understanding, practical tips	places_search, places_nearby, weather_forecast
weather	Outfit & activity advisor	weather_forecast, places_nearby

App widgets

Each widget calls POST /agent with {"app": "<id>", "prompt": "..."} and streams the SSE response inline.

• Blog write form — "✍️ Writing Assistant" card; automatically injects the post title + content as context
• Places page — "🗺️ Places Assistant" (logged-in only); natural-language place discovery
• Weather page — "🌂 Weather Advisor"; appears after a forecast loads, pre-populates current temperature and conditions as context

Usage

// Register a custom app agent from any package
agent.RegisterAppAgent(&agent.AppAgent{
    ID:           "myapp",
    Name:         "My App Assistant",
    SystemPrompt: "You are a helpful assistant for myapp...",
    Tools:        []string{"search", "web_search"},
})

Client POST:

{ "prompt": "Suggest tags for this post", "app": "blog" }

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