docs: add price alert bot recipe (closes #100)

[BigNathan1]

What

Adds a cookbook recipe and runnable example for an "if XLM > X notify me"
price alert bot, built on the existing WebSocket price stream (#L018).

Changes

• docs/cookbook/alert-bot.md (new) — worked walkthrough covering the
  data flow, the price_update message shape, the bot code, run
  instructions, HMAC-signed notify forwarding, x402-gated streams, and the
  server-side webhook alternative. Includes a live-demo section
  (GraphiQL at http://localhost:3002/graphiql, the published OpenAPI
  reference, and the local ws://localhost:3002/ws stream).
• examples/alert-bot/alert-bot.ts (new) — a minimal runnable bot that
  connects to /ws, watches a pair, and fires when the price crosses the
  threshold. Follows the existing oracle-relay example conventions
  (--help/--once, env-driven config, guarded entrypoint) and reuses
  crossesThreshold, buildThresholdAlertPayload, and
  deliverJsonWithRetries from src/alerts so the bot and server agree
  on crossing semantics.
• examples/alert-bot/README.md (new) — environment table and run notes.
• package.json — adds the alert:bot script.
• .env.example — documents the ALERT_BOT_* variables.
• README.md — links the new example and cookbook page.

Acceptance criteria

• Worked example: yes — both the cookbook page and a runnable
  npm run alert:bot.
• Live demo link: yes — GraphiQL IDE, published OpenAPI reference, and the
  local /ws stream.

Verification

Validated end-to-end with a temporary smoke harness (mirroring
scripts/ws-smoke.ts): a non-crossing update was ignored and a
0.14 -> 0.16 crossing fired ALERT XLM/USDC is above 0.15. No
production source files were changed.

closes #100

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