Add notifier driver: `discord` (webhook)

dotagent ships built-in notifier drivers under crates/dotagent-notify/ for desktop, slack, imessage, ntfy, pushover. Discord is a common ask for hobby projects, gaming clans, and small teams that don't run Slack.

Proposal

A new in-process driver discord that posts to a Discord webhook URL.

Manifest shape:

[[notifiers]]
driver = "discord"
webhook_url = "https://discord.com/api/webhooks/..."
username = "dotagent"          # optional override
avatar_url = "https://..."     # optional

The driver posts to the webhook with a Discord-formatted embed containing agent name, schedule, exit code, stderr tail. Same lifecycle as the other built-in notifiers — runs in-process inside the daemon.

Acceptance criteria

• Posts to a Discord webhook and renders correctly in the Discord channel.
• Uses reqwest with rustls-tls (no OpenSSL — repo convention).
• [security] allow-list works: network = ["discord.com"] plays nice.
• Validation rejects empty webhook_url with a clear error.
• Doc page added under docs/plugins/notify-discord.md (or docs/concepts/notifications.md extended).
• Example in examples/ or a snippet in the doc.

Where to start

• crates/dotagent-notify/src/slack.rs — closest analogue (also a webhook driver).
• crates/dotagent-notify/src/lib.rs — driver registration.
• Discord webhook docs: https://discord.com/developers/docs/resources/webhook#execute-webhook (rough shape, not a build instruction).

Non-goals

• Bot token auth. Webhook only for v1.
• Slash commands, threads, role mentions. Plain message is enough.