Capture hung-child stacks (SIGABRT) and slim pool-child startup

[maximusunc]

Diagnosing the omnicorp "hang" showed the child never reached the overlay ("Start omnicorp" never logged) -- it was wedged in its own cold startup, which a spawned pool child runs before every task (the worker is launched as a script, so spawn re-imports worker.py top-to-bottom). Two changes:

1. Kill stuck children with SIGABRT before SIGKILL. faulthandler (already enabled per child) catches SIGABRT and dumps the child's C+Python traceback to stderr, so the next hang leaves an actionable stack instead of a silent kill. A 0.5s grace lets the dump land; anything still alive (ignored SIGABRT) is then SIGKILLed so the slot is always freed.

2. Stop pool children from re-running the parent's startup work:

   • otel.setup_tracer now no-ops in spawned children (they emit no spans -- the task span lives in the parent) and imports the OTLP/grpc stack lazily, so a child never pays the grpc init / cold-start stall.
   • logger only attaches the RotatingFileHandler in the main process. Locally every child re-ran setup_logging and attached the handler, so N processes drove one non-multiprocess-safe handler on the same bind-mounted file -- a prime suspect for the local stall (prod is console-only via the Kubernetes branch, so this was local-only). Children now log to console.

Both use current_process().name, which spawn sets before re-importing the worker module, so it's reliable at import time. This removes the two most likely cold-start hang sources and instruments the path so the next occurrence is self-explaining.

Claude-Session: https://claude.ai/code/session_01TiiFCcToq8LgiEeaQhQM5e

[codecov]

Codecov Report

❌ Patch coverage is 48.64865% with 19 lines in your changes missing coverage. Please review.
✅ Project coverage is 51.04%. Comparing base (0b6be20) to head (ac19722).
⚠️ Report is 20 commits behind head on main.

Files with missing lines	Patch %	Lines
shepherd_utils/otel.py	0.00%	10 Missing ⚠️
shepherd_utils/process_pool.py	59.09%	6 Missing and 3 partials ⚠️

Files with missing lines	Coverage Δ
shepherd_utils/logger.py	100.00% <100.00%> (ø)
shepherd_utils/process_pool.py	70.52% <59.09%> (-14.09%)	⬇️
shepherd_utils/otel.py	0.00% <0.00%> (ø)

... and 6 files with indirect coverage changes

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