feat(matrix): Add multi-account support to Matrix channel

The Matrix channel previously hardcoded `listMatrixAccountIds` to always
return only `DEFAULT_ACCOUNT_ID`, ignoring any accounts configured in
`channels.matrix.accounts`. This prevented running multiple Matrix bot
accounts simultaneously.

Changes:
- Update `listMatrixAccountIds` to read from `channels.matrix.accounts`
  config, falling back to `DEFAULT_ACCOUNT_ID` for legacy single-account
  configurations
- Add `resolveMatrixConfigForAccount` to resolve config for a specific
  account ID, merging account-specific values with top-level defaults
- Update `resolveMatrixAccount` to use account-specific config when
  available
- The multi-account config structure (channels.matrix.accounts) was not
  defined in the MatrixConfig type, causing TypeScript to not recognize
  the field. Added the accounts field to properly type the multi-account
  configuration.
- Add stopSharedClientForAccount() to stop only the specific account's
  client instead of all clients when an account shuts down
- Wrap dynamic import in try/finally to prevent startup mutex deadlock
  if the import fails
- Pass accountId to resolveSharedMatrixClient(), resolveMatrixAuth(),
  and createMatrixClient() to ensure the correct account's credentials
  are used for outbound messages
- Add accountId parameter to resolveMediaMaxBytes to check account-specific
  config before falling back to top-level config
- Maintain backward compatibility with existing single-account setups

This follows the same pattern already used by the WhatsApp channel for
multi-account support.

Fixes #3165
Fixes #3085

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

docs/channels/matrix.md

@@ -136,6 +136,47 @@ When E2EE is enabled, the bot will request verification from your other sessions
 Open Element (or another client) and approve the verification request to establish trust.
 Once verified, the bot can decrypt messages in encrypted rooms.
 
+## Multi-account
+
+Multi-account support: use `channels.matrix.accounts` with per-account credentials and optional `name`. See [`gateway/configuration`](/gateway/configuration#telegramaccounts--discordaccounts--slackaccounts--signalaccounts--imessageaccounts) for the shared pattern.
+
+Each account runs as a separate Matrix user on any homeserver. Per-account config
+inherits from the top-level `channels.matrix` settings and can override any option
+(DM policy, groups, encryption, etc.).
+
+```json5
+{
+  channels: {
+    matrix: {
+      enabled: true,
+      dm: { policy: "pairing" },
+      accounts: {
+        assistant: {
+          name: "Main assistant",
+          homeserver: "https://matrix.example.org",
+          accessToken: "syt_assistant_***",
+          encryption: true,
+        },
+        alerts: {
+          name: "Alerts bot",
+          homeserver: "https://matrix.example.org",
+          accessToken: "syt_alerts_***",
+          dm: { policy: "allowlist", allowFrom: ["@admin:example.org"] },
+        },
+      },
+    },
+  },
+}
+```
+
+Notes:
+
+- Account startup is serialized to avoid race conditions with concurrent module imports.
+- Env variables (`MATRIX_HOMESERVER`, `MATRIX_ACCESS_TOKEN`, etc.) only apply to the **default** account.
+- Base channel settings (DM policy, group policy, mention gating, etc.) apply to all accounts unless overridden per account.
+- Use `bindings[].match.accountId` to route each account to a different agent.
+- Crypto state is stored per account + access token (separate key stores per account).
+
 ## Routing model
 
 - Replies always go back to Matrix.
@@ -256,4 +297,5 @@ Provider options:
 - `channels.matrix.mediaMaxMb`: inbound/outbound media cap (MB).
 - `channels.matrix.autoJoin`: invite handling (`always | allowlist | off`, default: always).
 - `channels.matrix.autoJoinAllowlist`: allowed room IDs/aliases for auto-join.
+- `channels.matrix.accounts`: multi-account configuration keyed by account ID (each account inherits top-level settings).
 - `channels.matrix.actions`: per-action tool gating (reactions/messages/pins/memberInfo/channelInfo).