agent: accept CR at the yes/no prompt

[michaelpersonal]

Problem

The Save current session? (y/n) prompt can hang. Typing y and pressing Enter
echoes ^M, the prompt never returns, and each further Enter appends another:

Save current session? (y/n) y^M^M^M

Cause

agent_prompt_yes_no_ex() reads with fgets(), which waits for '\n'.

The terminal may still be in linenoise raw mode at that point. editor_stop()
does leave cooked mode via linenoiseEditStop() → disableRawMode(), but
linenoiseRestoreRawMode() (driven by raw_mode_needs_restore) can put it back.
Raw mode clears ICRNL in linenoiseMakeRawMode(), so Enter arrives as '\r'
and never as '\n' — and fgets() blocks forever.

This is why it is intermittent rather than always reproducible.

Fix

Read the line by hand and terminate on '\r' or '\n'. Correct in both
modes: when ICRNL is enabled the tty has already translated CR, so a lone
'\r' cannot reach the application.

Raw mode also clears ECHO, so in that case neither the answer nor a newline is
echoed and following output runs into the prompt line. The patch emits a newline
itself, but only when ECHO is off, so cooked mode does not gain a blank line.
tcgetattr() fails on a non-tty, which correctly suppresses it for pipes too.

Testing

Driven through a pty on macOS (M5 Max, Metal), answering the save prompt with
CR only, never LF:

• before — prompt hangs, ^M accumulates, process must be killed
• after — answer accepted, 0 occurrences of ^M, clean exit

Builds warning-free under the existing -Wall -Wextra -std=c99. Adds
#include <termios.h>.