Replace the 200ms polling loop in chatd's execute and
process_output tools with server-side blocking via sync.Cond
on HeadTailBuffer.
The agent's GET /{id}/output endpoint accepts ?wait=true to
block until the process exits or a 5-minute server cap expires.
The process_output tool blocks by default for 10s (overridable
via wait_timeout), and falls back to a non-blocking snapshot on
timeout. The execute tool's foreground path makes a single
blocking call instead of polling.
Related #23316
The ampersand detection treated bash's pipe-stderr operator (|&)
as a trailing & for backgrounding, stripping it and producing a
broken pipe command. Also adds tests for execute.go and chatloop
context limit helpers, covering previously untested edge cases.
Mathias Fredriksson