[Feat] Let Fast attach Slack images to coding tasks

[roomote-roomote]

Opened on behalf of @daniel-lxs.

What changed

• Add an optional includeImages argument to launch_task and send_task_message, defaulting to false.
• When Fast explicitly opts in, pass supported images from the active conversation turn into new or existing coding tasks.
• Retain launch images in the task payload and deliver them with the coding harness's initial prompt.
• Deliver follow-up images through the existing active-task steering and resumable-task prompt paths.
• Require Slack's declared MIME type to be a supported prompt-image type, preventing unrelated files with misleading image extensions from being forwarded.

Why this change was made

Fast could inspect Slack images, but delegated coding tasks received only text. The explicit opt-in keeps attachment sharing voluntary while reusing the existing authenticated Slack download and task image contracts. It preserves the original bytes as data URLs without exposing Slack download URLs, bot tokens, or new cross-tenant attachment references.

Impact

Coding tasks can inspect multiple original Slack images at launch or in later Fast follow-ups when Fast sets includeImages to true. Omitted or false values keep the task message text-only, and attachments outside the active turn or unsupported image MIME types are not forwarded implicitly.

[roomote-community]

No new code issues found. See task

_{Reviewed 3d97165}

[roomote-roomote]

Updated launch_task so current-turn images are attached only when includeImages: true; omission defaults to text-only. Tool schema, prompt guidance, runtime plumbing, and focused tests now cover both opt-in and default-off behavior.

Shipped in cb5493b.