[Feat] Add Goal Mode across communication providers

[roomote-roomote]

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

What changed

• Added /goal <objective> activation to active Slack, Discord, Microsoft Teams, and Telegram task conversations.
• Registered native /goal commands for Discord and Telegram while preserving each provider's existing command and addressing semantics.
• Centralized the shared command identity, canonical text parsing, objective-only queue shaping, activation messages, and two-phase Goal Mode activation lifecycle.
• Carried trusted goal-generation context through communication queues into worker follow-up delivery so the first goal turn can safely complete or block the correct generation.
• Kept provider-specific dedupe, acting-user synchronization, thread routing, command option formats, and mock command-menu coverage intact.

Why this change was made

Goal Mode could previously be activated only from the web composer. Communication users should be able to use the same bounded continuation behavior without leaving their active task conversation, while avoiding another provider-specific implementation of the lifecycle itself.

Impact

Users can enable Goal Mode from every supported communication provider. The literal /goal invocation and provider thread history stay out of the agent-visible prompt; only the objective is delivered with trusted generation context. Failed or duplicate delivery does not commit the new goal state. Browser proof is not applicable because the changed behavior lives in provider transports and runtime queues rather than the configured web surface.

[roomote-community]

No code issues found. See task

_{Reviewed 14d1b0f}

[roomote-roomote]

No changes were needed on PR #1234: live review state has no unresolved threads and all CI checks are green. The reported review and CI failures belong to PR #1245, so the fixer moved there.