Keep unified exec output collection bounded

[copyberry]

Keep unified exec output collection bounded

Why

Unified exec can drain process output multiple times while waiting for a command. Collecting those drains into an uncapped buffer allows large commands to exceed the output collection limit.

What changed

• Accumulate drained output through the capped head/tail buffer so collection remains bounded while preserving the beginning and end of the stream.
• Include a ... N bytes omitted ... marker when the collection cap drops output.
• Calculate the original token estimate from all observed bytes and preserve omission metadata through truncation and sandbox-denial responses.

Testing

• Cover repeated drains, previously omitted output, omission markers, and large-output summaries.