fix(plugins): serialize plugin loader import critical-section (partial-import race)

[Kyzcreig]

Fixes an intermittent, silent partial-import race in the three plugin loaders (context_engine, memory, cron_providers).

Symptom

Intermittent WARNING run_agent: Context engine "lcm" not found — falling back to built-in compressor (and the identical mem0 provider signature), always preceded by delegate_tool concurrency warnings. Live evidence: on a bpx session the built-in compressor (not LCM) emitted the summary that fired the CLI reseed; lcm_status showed compression_count: 0 because LCM was never the active engine that turn.

Root cause

Each loader does sys.modules[module_name] = mod before spec.loader.exec_module(mod). Child agents run concurrently in a shared-process DaemonThreadPoolExecutor (delegate_task, max_concurrent_children) and share one sys.modules. A second concurrent caller sees the module in sys.modules (the if module_name in sys.modules short-circuit), grabs the half-initialized shell — register() / the engine class not defined yet — finds no engine, and falls back. The memory loaders getattr(cached, "__file__", None) guard does NOT help: module_from_spec sets __file__ before exec_module runs.

Fix

Wrap each loaders import critical-section in a module-level threading.RLock. A concurrent caller now either waits for the full load or observes a fully-initialized module — never a half-exec shell. Minimal diff: inner body renamed to *_locked, thin locking wrapper keeps the public name. RLock (not Lock) tolerates any reentrant load during exec.

Test (behavior contract, not snapshot)

tests/context_engine/test_loader_import_race.py reproduces the race deterministically: a coordination Event is set from inside the plugins module-top-level (i.e. mid-exec_module, after sys.modules is populated), so the racing thread is guaranteed to enter the vulnerable window. Validated fail-before/pass-after — with the lock bypassed the test FAILS with the exact production symptom (loaded but no engine instance found, B gets None); with the lock active it passes. 430/430 green across the three plugin-loader suites (context_engine, memory-provider, cron scheduler-provider).