[Docs] Unify Fast behavior across chat providers

[roomote-roomote]

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

What changed

• centralizes shared Fast context, delegation, web-link, and provider-conversation continuation behavior in the Sessions guide
• replaces the obsolete automation continuation matrix with one accurate contract for Slack, Discord, Microsoft Teams, and Telegram
• removes repeated shared capabilities and automation delivery prose from provider setup pages while retaining provider-specific commands, entry rules, attachments, setup, and UI limitations
• removes the stale Telegram web-only continuation caveat from the custom automation editor and covers the shared copy with a Telegram render test
• corrects Discord setup verification and automation report wording so neither assumes every Fast request immediately creates a task or thread

Why this change was made

Fast initiation and continuation now have provider parity, but documentation and automation settings still described the rollout's earlier provider differences. Keeping shared behavior in one place makes the current model easier to understand and prevents provider pages from drifting apart again.

Impact

Users now see one concise Fast behavior contract across supported communications providers, while real differences such as Slack !fast, Discord and Telegram /new, mention/thread/topic rules, Teams text-only request_user_input, attachment capabilities, and provider setup remain documented where they apply.

Mintlify validation and broken-link checks pass, the automation settings render suite passes all 50 tests, full lint and web typecheck pass, and the repository pre-push suite passes. Source searches found no remaining obsolete continuation wording outside historical release notes. Product-settings browser proof was blocked by the local setup guard, but the exact Telegram copy is covered by the render suite.

Screenshots

[roomote-community]

1 issue outstanding. See task

• apps/docs/automations.mdx:130 advertises Telegram topic destinations for Fast automations, but only Telegram chat targets are configurable and routed.

_{Reviewed 92f855a}

[roomote-community]

Telegram custom automation destinations are only chat IDs: the editor accepts a "Telegram chat ID" and buildFastAutomationConversation creates the Telegram reply target without a threadId. Therefore a Fast automation cannot be configured to deliver to a Telegram topic, despite this sentence. Please remove topics here (or add topic selection/routing support) so the shared contract does not advertise an unavailable destination.