#12468: feat(mattermost): add read, search, and channel-list actions

## Summary Adds message actions for querying Mattermost channels - useful for agents that need to catch up on channel history or search for context. ## Changes - `read`: fetch recent posts from a channel - `search`: search posts across channels with filters (author, date range, etc.) - `channel-list`: list available channels in a team Also fixes a bug where account-level `requireMention: false` was ignored (the setting wasn't passed through to the channel resolver). ## Testing - `pnpm check` - `pnpm vitest extensions/mattermost/src/actions.test.ts extensions/mattermost/src/channel.test.ts` <!-- greptile_comment --> <h2>Greptile Overview</h2> <h3>Greptile Summary</h3> Adds a Mattermost `actions` adapter to expose message-oriented tools (`read`, `search`, `channel-list`, `channel-info`) backed by new Mattermost client helpers (`fetchChannelPosts`, `searchPosts`, `fetchTeamChannels`). Also extends Mattermost account config/schema to support per-action toggles and fixes mention-gating behavior by passing the resolved account-level `requireMention` into the group require-mention resolver in the monitor. These changes integrate with the existing channel plugin by wiring `mattermostMessageActions` into `extensions/mattermost/src/channel.ts` and leveraging existing account resolution/config merge behavior in `extensions/mattermost/src/mattermost/accounts.ts`. <h3>Confidence Score: 5/5</h3> - This PR is safe to merge with minimal risk. - Changes are localized to the Mattermost extension, include tests for the new client helpers and action adapter behavior, and the mention-gating fix aligns with how the account resolver computes `requireMention`. I did not find any introduced logic errors that would break existing flows. - No files require special attention <!-- greptile_other_comments_section --> _{(3/5) Reply to the agent's comments like "Can you suggest a fix for this @greptileai?" or ask follow-up questions!} <!-- /greptile_comment -->