Start local PTY shells in a new process session and make close() escalate from SIGTERM to SIGKILL when a foreground command refuses to exit. This keeps code_execution_tool reset from blocking indefinitely on stuck terminal processes.\n\nAdd regression coverage for closing a TERM-resistant shell and document the reset lifecycle contract.
The code execution tool runs commands inside TTY-backed shells (local PTY and
remote SSH). Commands like `git diff`/`git log` detect the TTY and pipe output
through a pager (more/less). These shells never receive interactive input, so
the pager blocks forever and spins at 100% CPU per process — on a 16-core host
5 pager processes pegged 5 cores for 8+ hours (#1697).
Disable pagers in both session types:
- LocalInteractiveSession: inject PAGER=cat / GIT_PAGER=cat into the TTY env
- SSHInteractiveSession: export the same in the initial shell command
`cat` streams the output through instead of blocking, and also covers other
pager-using tools (man, systemctl, journalctl). Adds regression tests.
Fixes#1697
