[Feat] Let Fast respond to reactions 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

• Lets linked Fast session owners react to Roomote-authored replies in Slack, Discord, Microsoft Teams, and Telegram. Supported events enter Fast through one normalized optional external-input contract, so Fast may post a text response or stay silent.
• Resolves every reaction through an exact durable provider-message binding and verifies the linked reactor owns that Fast session before exposing conversation tools or active tasks.
• Preserves existing suggestion-card and configured Call Roomote via emoji precedence. Reaction removals do not trigger Fast, and Slack now rejects reaction side effects at runtime during every platform-event turn.
• Documents provider limits: Teams Bot Framework reports reactions only on Roomote messages, Discord reaction events omit target message text, and Telegram aggregate message_reaction_count updates lack reactor identity and remain unsupported.
• Adds focused provider, ownership, normalization, runtime-enforcement, provider-binding, removal, documentation, and regression coverage, plus a patch changeset.

Why this change was made

Slack already accepted reactions on Fast replies, but prompt-only guidance could not prevent an invalid second reaction, and the other communication providers stopped at suggestion or configured-emoji behavior despite exposing inbound reaction events. Provider-neutral awareness makes reactions consistent without weakening identity or conversation ownership boundaries.

Impact

Roomote can now treat reactions on its Fast replies as conversation input across every supported chat provider. Only newly added, user-attributed reactions on messages bound to the reactor's own Fast session qualify; existing suggestion launches and configured emoji automations keep their prior behavior.

Focused suites passed 337 tests across API, cloud agents, communication, and SDK packages. Package typechecks, full lint, docs validation and broken-link checks, git diff --check, and the repository pre-push suite also passed.

Screenshots

[roomote-community]

No new code issues found. See task

_{Reviewed c151126}