Nix-built hermes only copied skills/ into the output, so bundled platform
plugins weren't discoverable when running `nix run` (IRC invisible, no
plugin.yaml files present). Mirror the bundled-skills pattern:
- packages.nix: cleanSourceWith plugins/, copy to
$out/share/hermes-agent/plugins, set HERMES_BUNDLED_PLUGINS on every
wrapper.
- checks.nix: new bundled-plugins check verifying the directory, a
sample manifest, and the wrapper env var.
- hermes_cli.plugins.get_bundled_plugins_dir(): central helper that
honors HERMES_BUNDLED_PLUGINS with a dev-checkout fallback. Used by
plugins.py, plugins_cmd.py, gateway.py, and web_server.py so every
call site resolves the same path.
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