fix(warp): stop /dev/tty ENXIO error leaking to the user

[xpepper]

Summary

• emit-terminal-sequence.sh writes to /dev/tty as a fallback when CLAUDE_CODE_VERSION can't be resolved (e.g. during SessionStart, before the version-detection env caching kicks in). When the hook subprocess has no controlling terminal, opening /dev/tty fails with ENXIO, and bash reports "line N: /dev/tty: Device not configured" itself — to the original stderr, before the script's own 2>/dev/null on that same command takes effect. The result is a raw bash error printed to the user on session start, even though the script goes on to correctly fall back to the terminalSequence JSON output.
• Fix: wrap the write in a brace group before applying 2>/dev/null, so the suppression covers the whole group (including the redirect's own open failure) rather than trying to suppress errors on a redirect that fails before the following one applies.
• No behavior change otherwise — return codes and the JSON fallback path are identical.

Repro

$ bash -c 'printf hi > /dev/tty 2>/dev/null'
bash: line 1: /dev/tty: Device not configured   # leaks despite 2>/dev/null

$ bash -c '{ printf hi > /dev/tty; } 2>/dev/null'
# (nothing — cleanly suppressed)

Before/after against this repo's emit_terminal_sequence, no controlling tty, unset CLAUDE_CODE_VERSION:

# before
./emit-terminal-sequence.sh: line 82: /dev/tty: Device not configured
{"terminalSequence":"test-seq"}

# after
{"terminalSequence":"test-seq"}

Test plan

• plugins/warp/tests/test-hooks.sh: 57/57 passing, unchanged
• Manual before/after repro (above) confirming the leaked error is gone and the JSON fallback output is unchanged