[Feat] Add Fast orchestrator to Discord

[roomote-roomote]

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

What changed

• Added /fast request:<request> as a top-level Discord orchestrator entry point that works without an active task.
• Let the shared fast agent answer directly or launch a fresh Discord task thread, with Discord-specific lifecycle guidance that excludes Slack reactions and sender-message instructions.
• Registered the Discord command and aligned help text, public documentation, mock runbooks, examples, and eval scenarios with the slash-interaction flow.
• Made /fast the canonical Slack spelling while retaining !fast as a compatibility alias.
• Integrated the current Slack fast-default behavior without extending preference-driven routing to Discord.
• Kept Fast mode out of the web task composer until a dedicated top-level web experience is designed.

Why this change was made

Fast mode is an orchestrator that decides whether to answer or delegate work, not a mode entered inside an existing task. Discord should expose that same product boundary as Slack while keeping Slack's preference-driven default behavior scoped to the surface it was designed for.

Impact

Linked Discord users can invoke /fast from a DM, channel, or thread, receive a direct answer when no task is needed, and get a new task thread when execution is appropriate. Slack users keep explicit /fast and legacy !fast, and eligible Slack messages can still default to Fast mode when the deployment and user settings are enabled. Discord messages do not opt into that preference-driven default. Browser proof is not applicable because the shipped behavior lives in communications transport and orchestration rather than the web product surface.

[roomote-community]

No code issues found. See task

• Web /fast now includes visible task conversation context.
• Concurrent web /fast calls now receive unique transcript timestamps.
• Web /fast records task activity at a far-future timestamp.
• Web /fast bounds transcript history in the database.
• Web /fast excludes unflagged legacy workflow and environment wrapper prompts before the database limit.
• Web /fast pages past legacy Slack thread_activity-only prompts before filling its visible context.
• Web /fast can duplicate and omit visible messages when task activity inserts envelopes between the independent OFFSET page queries (the web /fast implementation was removed from this PR).
• Reconciled Discord /fast with Slack-only default routing and sender-specific Fast-agent guidance.

_{Reviewed b953b26}

[roomote-roomote]

Addressed the full review sequence through e1aee1b.

• Web /fast fills its visible context by paging through bounded database result sets and applying the shared transcript visibility resolver to every candidate.
• Explicitly hidden rows, legacy wrapper prompts, and Slack thread_activity-only prompts cannot consume the final 500-message context allowance.
• Concurrent calls receive atomic, millisecond-scale timestamp pairs without overwriting envelopes or skewing task activity.
• Focused database/server tests, affected package typechecks, and the full pre-push suite pass.
• All review threads are resolved and PR metadata is refreshed.

[roomote-roomote]

Updated the branch through b953b26d.

• Merged current develop, including the Slack fast-default behavior and later Fast-mode fixes, without rewriting branch history.
• Kept preference-driven Fast routing Slack-only; Discord /fast remains an explicit top-level orchestrator.
• Preserved Slack /fast as canonical and !fast as a compatibility alias.
• Scoped Slack sender and reaction prompt guidance away from Discord.
• Focused API, cloud-agent, and communication tests passed (165 tests total); affected TypeScript checks and changed-file lint passed.
• The repository pre-push suite passed during validation and again during push.
• PR metadata is refreshed; existing review threads remain resolved.