#10587: fix(extensions/mattermost): pass requireMention override from chatmode config

## Problem `chatmode: "onmessage"` config doesn't work — bot still requires @mention in channels. ## Solution Propagate `account.requireMention` to `resolveRequireMention()` so the chatmode setting is respected. <!-- greptile_comment --> <h2>Greptile Overview</h2> <h3>Greptile Summary</h3> - Fixes Mattermost mention-gating by passing the account-level `requireMention` (derived from `chatmode`) into `core.channel.groups.resolveRequireMention`. - This makes `chatmode: "onmessage"` correctly disable the “must @mention the bot in channels” requirement. - Change is localized to inbound message handling in `extensions/mattermost/src/mattermost/monitor.ts` and uses the existing `requireMentionOverride` hook in core group-policy resolution. <h3>Confidence Score: 5/5</h3> - This PR is safe to merge with minimal risk. - The change is a single-parameter propagation into an existing core API (`requireMentionOverride`) and matches the expected type/semantics, making Mattermost `chatmode` correctly affect mention gating in channels without altering unrelated routing or dispatch logic. - extensions/mattermost/src/mattermost/monitor.ts <!-- greptile_other_comments_section --> _{(2/5) Greptile learns from your feedback when you react with thumbs up/down!} <!-- /greptile_comment -->