[Fix] Revert high-latency skill discovery commands across providers

[roomote-roomote]

​Opened on behalf of @daniel-lxs. Follow up by mentioning @roomote-roomote, in the web UI, or in Telegram.

What changed

Revert the complete merged change from PR #2999, removing skill discovery commands and their supporting catalog logic across Slack, Discord, Telegram, Microsoft Teams, and AgentMail. This also removes the related command registrations, tests, and public documentation. Retired /skills interactions from existing Discord installations now overwrite the global command set and, for guild-scoped commands, the affected guild command set, then return a clear ephemeral retirement message.

Why this change was made

The shipped skill discovery path introduced high latency while building remote skill catalogs and created substantial follow-up complexity. This rollback restores the pre-#2999 behavior while preserving unrelated changes already on develop; the stale-command guard prevents existing Discord registrations from becoming silent dead ends during rollout.

Impact

Users will no longer receive /skills or skills discovery commands through the communication providers covered by #2999. Existing exact skill invocation and unrelated communication workflows remain unchanged. Existing Discord users with a stale /skills picker entry receive an explicit retirement response while both the global registration and, when applicable, the guild registration are refreshed. Open PR #3002 is an unmerged follow-up based on #2999; it is intentionally not modified or closed, but it will require reconsideration because this revert removes the feature it is designed to improve.

[roomote-community]

No code issues found. See task

• Guild-scoped registration does not remove the globally registered /skills command, so affected server command menus remain stale.

_{Reviewed cf83026}

[roomote-roomote]

Resolved the outstanding Discord registration issue from the latest Roomote code review.

• Stale /skills interactions now always refresh the global command registration; guild-scoped interactions also refresh their interaction guild_id registration.
• The interaction receives an ephemeral retirement response even if either registration request fails.
• The [Feat] Add skill discovery commands across communication providers #2999 skill discovery implementation remains fully reverted; no catalog or invocation behavior was restored.
• Regression coverage asserts both global and guild registration scopes.
• Commit: cf830260
• Validation: Discord API handler tests (71 passed), API fast typecheck, oxlint, and pre-push lint/typecheck/Knip gates passed.
• The Roomote code review check passed on the pushed commit.