Add discord webhook support

Support posting activity to a plain Discord webhook URL, as an alternative to the gateway bot.

Grounded / scope check: tesks already posts Discord activity natively via the gateway bot — DiscordActivitySink → bot.postEmbed(channel, embed), configured per guild (PAR-72/PAR-74). This issue predates that, so confirm it's still wanted.

If so, the remaining value is supporting incoming-webhook URLs (no bot token / gateway needed): add a webhook-URL field to the guild/channel config and an ActivitySink variant that POSTs the embed JSON to the URL via HttpClient (same shape as the GitHub write-back client). Useful for servers that don't want to invite the bot. Otherwise close as superseded by native posting.