Allow cross-channel Slcak posting when channel is explicitly identified in prompt

Description:
When a user references a specific Slack channel in their prompt (e.g., "post this update to #CCY8M69PZ"), Sam should be able to post to that channel even when the originating prompt comes from a different channel.

Current behavior:​ Sam can only post to the channel where the session originated. If a user asks Sam to post to another channel, Sam acknowledges the request but fails silently or acknowledges it can't follow through — creating a trust gap.

Expected behavior:​ If a target channel is explicitly named in the prompt, Sam should be able to post there (subject to appropriate permissions).

Use case:​ Analytics team uses Sam across multiple channels. Users frequently ask Sam to post updates to originating Jira/project threads, pipeline channels, or stakeholder channels based on context — not just the channel where they're prompting.