plugins/context_engine.load_context_engine scanned only the bundled directory. An engine
dropped into $HERMES_HOME/plugins/<name> with `context.engine: <name>` was reachable only
through the general plugin system, which skips any user plugin not listed in
plugins.enabled — so every agent init logged "Context engine '<name>' not found — falling
back to built-in compressor" although the engine was installed and named in config.
Live probe on base (fake HOME, plugins/ctx_demo with register(ctx), context.engine:
ctx_demo): the warning fired on EVERY init, not only the first; adding the plugin to
plugins.enabled made it load through the general fallback. `context.engine` is the
activation signal (as memory.provider / cron.provider are for their kinds), so the engine
loader now resolves bundled then user dirs the way plugins/cron_providers does: same
`user_plugins_dir()` seam, cheap source heuristic (register_context_engine / ContextEngine),
user engines imported under a synthetic namespace, bundled wins on collision, and
discover_context_engines() lists them for `hermes plugins` / the dashboard.
Fixes#61839
credit: @giggling-ginger #61995
913c045c53