refactor: extract subprocess cleanup into shared subproc helper

[iliaal]

Summary

bird_x.py and youtube_yt.py had four near-identical copies of the same subprocess cleanup dance. Each callsite did the same thing: spawn in a new process group via os.setsid, call communicate(timeout=N), catch TimeoutExpired, signal SIGTERM to the group via killpg, fall back to proc.kill() on signal errors, wait up to 5 seconds, then return or log an error.

I extracted it to scripts/lib/subproc.run_with_timeout():

• Runs the child in its own process group via os.setsid where available
• Raises SubprocTimeout on timeout
• On timeout: SIGTERM the group, fall back to proc.kill(), wait up to 5s, raise
• Accepts an on_pid callback so bird_x.py can still register child PIDs with last30days.register_child_pid for whole-process cleanup tracking
• Captures stdout and stderr as strings in a SubprocResult dataclass

With the shared helper in place, import signal and import subprocess became dead in both source files (plus import os in youtube_yt.py) and went with them.

Call sites migrated

• bird_x._run_bird_search() (Bird Node.js CLI)
• bird_x.search_handles() inner worker (per-handle Bird call)
• youtube_yt.search_youtube() (yt-dlp search)
• youtube_yt.fetch_transcript() (yt-dlp per-video transcript download)

Diff summary

• scripts/lib/subproc.py: +94 new file
• scripts/lib/bird_x.py: -51 net
• scripts/lib/youtube_yt.py: -34 net
• tests/test_subproc.py: +100 new file, 9 tests covering success, non-zero exit, stderr capture, timeout-raises, timeout-kills-group, missing-command, env passthrough, PID callback, callback-exception-suppression
• tests/test_env_v3.py: +3 / -1 (patch subproc.run_with_timeout instead of the removed bird_x.subprocess)
• tests/test_youtube_yt.py: +8 / -6 (same adjustment for yt-dlp flag assertions)

Test plan

• 9 new subproc tests pass (full branch coverage of the helper)
• All 7 bird_x tests pass
• All 30 youtube_yt tests pass
• Full suite: 1031 passed, 15 pre-existing failures (unchanged)
• Live smoke test: X search returned 16 posts via Bird
• Live smoke test: YouTube search found 5 videos for "React 19 tutorial" and fetched transcripts for all 5
• I verified the timeout path: the first YouTube query stalled past 120s, SubprocTimeout fired, the retry with a different query succeeded.