v0.381.1

Fixes a long-standing CI hang documented in .local/BUGS.md as
"CI hang #2: tests/run_migrated.clj on ubuntu-24.04". The hang was
deterministic on GHA ubuntu-24.04 runners (both x86 and arm) and
masked on macos-14 plus Apple Silicon Docker. Reproducer: a single
(dosync (send a fn)) followed by (await a) would block forever
on the very first iteration.

Root cause

agent_worker_ensure released agent_mu between spawning the
worker pthread and the caller's subsequent agent_enqueue call.
A freshly-spawned worker could win the race to agent_mu, see an
empty runq, set worker_alive = 0, and exit before the producer
ever published its action. The producer's later agent_enqueue
then pushed the node + bumped in_flight into a runq with no
consumer; (await a) waited on a condition that nothing would
ever signal.

Fix

Merge the worker spawn and the enqueue into a single
agent_enqueue critical section. The producer:

1. Takes agent_mu.
2. Decides whether a worker is needed (worker_alive == 0), and
   if so reserves a thread-budget slot atomically.
3. Pushes the action node and bumps in_flight (still under
   agent_mu).
4. Releases agent_mu.
5. Calls pthread_create only after the node is published.

The new worker's first agent_mu_lock therefore always observes
a non-empty runq for the producer that spawned it. Thread-budget
refusals revert the enqueue and free the node before throwing so
(await a) is never stranded on an action that no worker will
process.

The dead agent_worker_ensure and its inline spawn body are
removed. agent_worker_reap_pending stays — mino_agent_quiesce_workers
still uses it on state teardown, and the new agent_enqueue calls
it under agent_mu (with a state-lock yield around the
pthread_join) when the previous worker has exited.

Regression test

Added tests/migrated/dosync_send_repro.clj to mino-tests —
five iterations of the dosync-send-await pattern with explicit
progress logging. Pre-fix, this hangs after the first
[repro] iter 0 before await on Linux CI. Post-fix, all five
iterations complete in ~50ms locally and the full Migrated suite
passes on every CI runner.

Verification

• Full mino test suite green (1371 tests, 4828 assertions).
• Local linux/amd64 + linux/arm64 Docker run the migrated suite
  in ~24s end-to-end; isolated repro completes in 50ms.