fix: handle CR in agent yes/no prompts

[arsis-dev]

Summary

ds4-agent now treats carriage return as a valid line terminator in its yes/no prompts.

This fixes an interactive exit path where a terminal can send \r after replying to:

Save current session? (y/n)

With the old line-buffered parser, that response could remain visually stuck as y^M instead of being accepted cleanly.

Root cause

The prompt helper parsed complete newline-delimited lines with fgets(). Some terminal paths can deliver carriage return for Enter in this restored-terminal prompt path.

Upstream now also uses the same prompt helper for timed approval prompts, so the fix keeps that timeout behavior while switching the answer parser to read one byte at a time.

Change

agent_prompt_yes_no_ex() now:

• records the first non-space answer byte;
• accepts either \n or \r as the end of the response;
• preserves EOF / Ctrl+D as a negative answer;
• preserves automatic timeout answers for callers that opt into them.

Validation

Current branch after rebase onto origin/main (ad0209f):

git diff --check origin/main..HEAD
make ds4-agent
make ds4_test
./ds4_test --server

Results:

• git diff --check origin/main..HEAD: passed.
• make ds4-agent: passed.
• make ds4_test: passed.
• ./ds4_test --server: passed.

Earlier, before this rebase, the same fix was also validated with a full make test from a normal macOS Terminal on Apple M5 Max / Metal backend. The Codex execution context used for this rebase cannot access a Metal device directly, so the current post-rebase verification uses the non-Metal server regression check.