SDK session init accepts a plugins field over stdin

The SDK's initialize control-request schema now accepts a plugins field, mirroring the SDK's plugins launch option, so callers can pass plugin-dir entries over stdin instead of adding one --plugin-dir flag per plugin. It only takes effect on a CLI launched with --await-initialize, where it's read during startup before plugin work happens; a repeated initialize call or a remote session transport ignores it. The response's plugins_applied field reports whether it actually took effect.