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fix(agtermctl): survive oversized requests instead of dying on SIGPIPE (#340)

* fix(agtermctl): survive oversized requests instead of dying on SIGPIPE

A request over the server's 1 MiB line cap made the server close the
connection mid-write; the client fd had no SO_NOSIGPIPE (the server side
sets it, with a comment naming this exact hazard), so agtermctl died on
signal 13 — exit 141, zero output. Reachable from ordinary use:
session type --stdin with a large file, a big pick list, a long
session restore.

Two changes, both needed: SO_NOSIGPIPE alone still loses the server's
"request too large" message, because writeAll throws on EPIPE before
readLine ever runs.

- set SO_NOSIGPIPE on the client fd after socket(), mirroring the server
  (Darwin-only, keeping the Glibc test build compiling)
- check the encoded request against the cap before connecting, so an
  oversized request fails with a readable error naming the size and cap;
  the newline is appended after the check, matching the server's
  newline-excluded count
- hoist the cap into agtermCore (ControlWire.maxRequestLineBytes) and
  point ControlServer at it, so the two sides can't drift

Tests: an oversized request throws before connecting; a hang-up stub
server proves a mid-write peer close surfaces as a thrown write error
rather than killing the process (removing the setsockopt makes the
runner die on signal 13, so the test has teeth).

* docs: single ControlWire doc carrying the sizing rationale; drop the contradicted Glibc clause

Review feedback on #340: the Darwin-guard comment claimed the Glibc build
never writes to a dead peer, which writeToHungUpPeerThrowsInsteadOfDying
in the same commit does; and the enum/member docs restated one contract
while losing the old cap-sizing rationale. One doc on ControlWire now
carries sizing + newline exclusion, matching OverlayResultError and
ControlNotify.

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