refactor: drop process-level --plugin-config in favor of config files

[zhongxuanwang-nv]

Overview

Drops the process-level --plugin-config ad-hoc CLI flag in favor of configuration files. Gateway plugin configuration should come from plugins.toml or an inline [plugins].config block, not from one-off command-line JSON.

• I confirm this contribution is my own work, or I have the right to submit it under this project's license.
• I searched existing issues and open pull requests, and this does not duplicate existing work.

Details

What changed

• Removed --plugin-config (and the NEMO_RELAY_PLUGIN_CONFIG env var) from nemo-relay serve (ServerArgs) and nemo-relay run (RunCommand).
• Removed apply_cli_plugin_config and the run-vs-inherited precedence shim that only existed because both structs carried the flag.
• Updated docs: the gateway now resolves plugin configuration from two mutually exclusive file source classes (plugins.toml or [plugins].config); removed the --plugin-config row, conflict clause, and the "use --plugin-config for automation" guidance.
• Removed the now-obsolete tests (the --plugin-config↔file conflict and the run-overrides-inherited shim) and trimmed the flag from the remaining run/serve config tests.

Unchanged (separate mechanism)

• The per-session nemo-relay hook-forward / install --plugin-config, which sets the x-nemo-relay-plugin-config session header, is a different feature (session metadata, not gateway activation) and is intentionally left in place.

Why
Steer gateway plugin configuration toward configuration files instead of ad-hoc command-line JSON, and simplify the precedence model (the process-level --plugin-config was one of three competing sources for the same GatewayConfig.plugin_config).

How tested

• cargo test -p nemo-relay-cli (0 failed), cargo clippy -- -D warnings, cargo fmt --check.

Breaking changes

• --plugin-config and NEMO_RELAY_PLUGIN_CONFIG are no longer accepted by nemo-relay serve or nemo-relay run. Use plugins.toml or [plugins].config instead.

Where should the reviewer start?

crates/cli/src/config.rs — the removal of the ServerArgs/RunCommand plugin_config fields, apply_cli_plugin_config, and the simplified resolve_run_config.

Related Issues: (use one of the action keywords Closes / Fixes / Resolves / Relates to)

• Relates to relay-183 (internal ticket; GitHub issues are disabled on this repo). Split out from the code-driven plugin config layer PR to keep that change scoped.