An attachment larger than the model's context breaks every later run in the chat #480

Found while adding PDF attachments (#479). Attachments are capped at 1.5 MB per message (the hosted-channel row cap), which counts bytes, not tokens. A large text file or PDF can fit under that cap and still exceed the model's context: Keynote's multilingual license PDF extracted to about 1 MB of text, about 466K tokens, over Haiku's 200K window.

Actual: that run fails, and every later run in the chat fails the same way until the message is deleted (#467 deletion replaces it in context).
Expected: the oversized attachment is refused or trimmed when sent, or the run explains what happened and the chat stays usable.

Possible directions: check the attachment's estimated tokens against the chosen model's context window where the channel knows it (pi-ai model metadata), or have the context edit drop or summarize an attachment that overflows.

Present since #469 (text attachments). Part of #26 and #5.