feat: scope group mention patterns by channel

Provider-scoped configured regex mention patterns for Discord, Matrix, Slack, Telegram, and WhatsApp.

Native platform mentions keep their existing behavior, and unsupported channels do not opt into the new regex policy path. The new policy supports per-channel allow/deny routing through mentionPatterns.mode with allowIn and denyIn so group auto-reply regexes can be limited without broad global blast radius.

Refs #70864.
Supersedes #87200.
Thanks @patrick-slimelab.

docs/.generated/config-baseline.sha256

@@ -1,4 +1,4 @@
-370da2e3a4253f00c3963a3ad8b57707ea3f67a8d0d394b7d2b96db4f3413d32  config-baseline.json
-6a66c70d36dacf5fd1a8b7e157d1ff4812e97f518c13ebc3190509df4c269f29  config-baseline.core.json
-a9102c0611b8170fac37853cc31771810f31757a9e3b2c6796bbd9625f9b9206  config-baseline.channel.json
-923a8cac695c752e51751cc2dea185a3fbe19d0015722f7ea1909f897dfbb898  config-baseline.plugin.json
+8162a661edc183008a336db265a092acc90762c6c547b1383ef14fd0d381dea5  config-baseline.json
+5ee177382cf32c2816dca0a4e67cd6c01df1045d600b21a6e9c11639ddb10ce8  config-baseline.core.json
+7a7aba829deb8b54047b5c9e7dd3f3c9eade721e2f728db339c4ea99b77a162e  config-baseline.channel.json
+e6a1d6f51f0d9c04bd92d51deebfaca8c7917dd28d7998d225c0074e0a095348  config-baseline.plugin.json

docs/.generated/plugin-sdk-api-baseline.sha256

@@ -1,2 +1,2 @@
-3cc84cf3d30697d541ba98a5c1835784a4254a9193e51b009372e1620948e430  plugin-sdk-api-baseline.json
-515c9e2972f0d79dbed27ffae815a96d432a341005046d603a82a235c4108340  plugin-sdk-api-baseline.jsonl
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+bf3e94dcccaf169811990dfd058a16ace8ce2ab44ea9eac6042b525b6d8baf5f  plugin-sdk-api-baseline.jsonl

docs/channels/groups.md

@@ -362,6 +362,8 @@ Replying to a bot message counts as an implicit mention when the channel support
   <Accordion title="Mention gating notes">
     - `mentionPatterns` are case-insensitive safe regex patterns; invalid patterns and unsafe nested-repetition forms are ignored.
     - Surfaces that provide explicit mentions still pass; patterns are a fallback.
+    - `channels.<channel>.mentionPatterns.mode: "deny"` disables configured mention patterns by default for that channel; opt selected conversations back in with `allowIn`.
+    - `channels.<channel>.mentionPatterns.denyIn` disables configured mention patterns for specific conversation IDs while native platform @mentions still pass.
     - Per-agent override: `agents.list[].groupChat.mentionPatterns` (useful when multiple agents share a group).
     - Mention gating is only enforced when mention detection is possible (native mentions or `mentionPatterns` are configured).
     - Allowlisting a group or sender does not disable mention gating; set that group's `requireMention` to `false` when all messages should trigger.