feat: add Mattermost channel support

Add Mattermost as a supported messaging channel with bot API and WebSocket integration. Includes channel state tracking (tint, summary, details), multi-account support, and delivery target routing. Update documentation and tests to include Mattermost alongside existing channels.

docs/channels/index.md

@@ -15,6 +15,7 @@ Text is supported everywhere; media and reactions vary by channel.
 - [Telegram](/channels/telegram) — Bot API via grammY; supports groups.
 - [Discord](/channels/discord) — Discord Bot API + Gateway; supports servers, channels, and DMs.
 - [Slack](/channels/slack) — Bolt SDK; workspace apps.
+- [Mattermost](/channels/mattermost) — Bot API + WebSocket; channels, groups, DMs.
 - [Signal](/channels/signal) — signal-cli; privacy-focused.
 - [BlueBubbles](/channels/bluebubbles) — **Recommended for iMessage**; uses the BlueBubbles macOS server REST API with full feature support (edit, unsend, effects, reactions, group management — edit currently broken on macOS 26 Tahoe).
 - [iMessage](/channels/imessage) — macOS only; native integration via imsg (legacy, consider BlueBubbles for new setups).

docs/channels/mattermost.md

@@ -0,0 +1,87 @@
+---
+summary: "Mattermost bot setup and Clawdbot config"
+read_when:
+  - Setting up Mattermost
+  - Debugging Mattermost routing
+---
+
+# Mattermost
+
+## Quick setup
+1) Create a Mattermost bot account and copy the **bot token**.
+2) Copy the Mattermost **base URL** (e.g., `https://chat.example.com`).
+3) Configure Clawdbot and start the gateway.
+
+Minimal config:
+```json5
+{
+  channels: {
+    mattermost: {
+      enabled: true,
+      botToken: "mm-token",
+      baseUrl: "https://chat.example.com"
+    }
+  }
+}
+```
+
+## Environment variables (default account)
+Set these on the gateway host if you prefer env vars:
+
+- `MATTERMOST_BOT_TOKEN=...`
+- `MATTERMOST_URL=https://chat.example.com`
+
+Env vars apply only to the **default** account (`default`). Other accounts must use config values.
+
+## Chat modes
+Mattermost responds to DMs automatically. Channel behavior is controlled by `chatmode`:
+
+- `oncall` (default): respond only when @mentioned in channels.
+- `onmessage`: respond to every channel message.
+- `onchar`: respond when a message starts with a trigger prefix.
+
+Config example:
+```json5
+{
+  channels: {
+    mattermost: {
+      chatmode: "onchar",
+      oncharPrefixes: [">", "!"]
+    }
+  }
+}
+```
+
+Notes:
+- `onchar` still responds to explicit @mentions.
+- `channels.mattermost.requireMention` is honored for legacy configs but `chatmode` is preferred.
+
+## Targets for outbound delivery
+Use these target formats with `clawdbot message send` or cron/webhooks:
+
+- `channel:<id>` for a channel
+- `user:<id>` for a DM
+- `@username` for a DM (resolved via the Mattermost API)
+
+Bare IDs are treated as channels.
+
+## Multi-account
+Mattermost supports multiple accounts under `channels.mattermost.accounts`:
+
+```json5
+{
+  channels: {
+    mattermost: {
+      accounts: {
+        default: { name: "Primary", botToken: "mm-token", baseUrl: "https://chat.example.com" },
+        alerts: { name: "Alerts", botToken: "mm-token-2", baseUrl: "https://alerts.example.com" }
+      }
+    }
+  }
+}
+```
+
+## Troubleshooting
+- No replies in channels: ensure the bot is in the channel and mention it (oncall), use a trigger prefix (onchar), or set `chatmode: "onmessage"`.
+- Auth errors: check the bot token, base URL, and whether the account is enabled.
+- Multi-account issues: env vars only apply to the `default` account.