[cablehead]

Adds the nu-cmd-plugin command context (0.116.0, matching the other nu crates) to the engine in Engine::new, so scripts can use plugin list, plugin stop, plugin use, plugin add and plugin rm. Previously these failed with "Command plugin not found".

The use case is a served script checking on a plugin loaded with --plugin, e.g. plugin list --engine | where name == foo | get pid.0 to see whether the plugin process exited and was respawned.

Changes:

• Cargo.toml / Cargo.lock: add nu-cmd-plugin = "0.116.0", locked at 0.116.0 so the other nu crates stay on 0.116.0.
• src/engine.rs: call nu_cmd_plugin::add_plugin_command_context.
• README.md: note in the Plugins section that the plugin commands are available.

Tests:

• server_test::test_plugin_list_engine: a served closure with the test plugin loaded; plugin list --engine reports status running and a pid equal to the one test-plugin-pid returns.
• eval_test::test_eval_plugin_stop_respawns: after plugin stop test, the next plugin call runs in a new process and plugin list --engine reports the new pid.

cargo fmt --check, cargo clippy (flags from scripts/check.sh), cargo test, the three tests/*.nu suites and the two example tests pass. deno fmt README.md --check was not run (deno not installed on the machine).