feat(slash-commands): add /weather and /exchange commands

[AnayDhawan]

What

Two new slash commands — /weather <city> and /exchange <FROM> <TO> [amount] — that run directly in the composer without invoking the AI model. Results are injected into the chat as local messages.

Why

Closes #249. Quick factual lookups shouldn't cost tokens or require waiting for a model response.

How

Added a local-run outcome type to SlashOutcome. When returned, composer.tsx awaits the execute function and injects user + result messages directly via Chat.messages setter, bypassing the transport layer entirely. API calls go through the existing ai_http_request Tauri command. /weather reads the OWM key from the OS keychain via secrets_get; /exchange uses frankfurter.app (no key needed).

Testing

• pnpm exec tsc --noEmit clean
• /weather Mumbai — returns conditions table
• /weather New York imperial — returns °F
• /exchange USD INR — returns rate
• /exchange USD INR 500 — returns converted amount
• /weather with no OWM key — returns helpful setup message

Notes for reviewer

/weather requires a free OpenWeatherMap key saved in Settings → API Keys (account: openweathermap). /exchange works out of the box.

[ResourceMint]

Lowkey a good idea to have this in the terminal.