fix: prevent shell tool hang with pager-based commands in non-interactive mode

[xlyoung]

Problem

When the shell tool runs commands like git diff or man in non-interactive mode, the spawned pager (usually less) hangs because:

1. TERM environment variable is not set → less reports WARNING: terminal is not fully functional
2. The pager waits for user input that never comes → command hangs until timeout

Root Cause

In execute_with_pty(), the child process doesn't set TERM or disable pagers:

if pid == 0:  # Child process
    os.chdir(cwd)
    os.execvp("/bin/sh", ["/bin/sh", "-c", command])  # TERM not set, pagers enabled

Fix

1. Set TERM=xterm-256color if not already set (needed for PTY emulation)
2. In non-interactive mode, disable pagers by setting GIT_PAGER=cat, PAGER=cat, MANPAGER=cat

if pid == 0:
    os.environ.setdefault("TERM", "xterm-256color")
    if non_interactive_mode:
        os.environ.setdefault("GIT_PAGER", "cat")
        os.environ.setdefault("PAGER", "cat")
        os.environ.setdefault("MANPAGER", "cat")

Testing

All 30 shell tests pass.

Fixes #360