A general-plugin context engine is one shared instance; agent init copied it per agent
with copy.deepcopy() only. Engines that hold a SQLite connection or lock (hermes-lcm)
already expose clone_for_agent() for exactly this, but it was never called, so every
init logged "could not be safely copied … falling back to built-in compressor" and the
engine was unusable through the plugin system.
ContextEngine grows clone_for_agent() (default: deepcopy, the previous behaviour) and
_select_context_engine calls it; the failure message now names the hook to override.
Docs: context-engine-plugin.md documents the per-agent clone contract.
Test change (existing on main): tests/agent/test_context_engine.py::
test_agent_init_source_deepcopies_singleton_not_aliases was a source-reading pin on the
literal `copy.deepcopy(_candidate)` line, which this fix intentionally replaces. It is
superseded by tests/agent/test_plugin_context_engine_clone.py, which drives the real
_select_context_engine seam and asserts the invariant it guarded (child update_model()
never mutates the shared singleton) plus the new clone_for_agent() path.
Fixes#99640
credit: @stephenschoettler #62374
credit: @686f6c61 #99677
