Add the `--node-linker [isolated|hoisted|pnp]` CLI flag to
`pacquet install`, mirroring pnpm's flag. Overrides
`Config.node_linker` for the invocation; absent flag → config's
yaml/npmrc value wins.

The CLI parses into a `NodeLinkerArg` mirror enum (kept in the
CLI crate so `pacquet-config` stays free of `clap`), then
`into_config()` converts to the canonical
`pacquet_config::NodeLinker`.

Threaded into the `Install` runner as an explicit
`node_linker: NodeLinker` field rather than reading
`config.node_linker` directly. Matches the existing pattern
`supported_architectures` uses (`state.config` is `&'static`, so
the override-merge happens at the CLI layer and lands here as a
fully-resolved value). `build_modules_manifest` consumes it on
the `.modules.yaml.nodeLinker` write so the persisted setting
reflects the invocation, not just the config.

`pacquet add` uses Config's value by default (`add` doesn't
expose `--node-linker` per the umbrella scope; Slice 6's pipeline
integration will revisit if needed).

5 new CLI tests:

- `node_linker_default_is_none` — flag absent → field is None.
- `node_linker_hoisted` / `node_linker_isolated` / `node_linker_pnp`
  — each value parses and round-trips through `into_config()`.
- `node_linker_invalid_value_rejected` — clap rejects unknown
  values with the bad value in the error message.
- `node_linker_arg_into_config_matches_every_variant` — every
  ValueEnum variant has a canonical mapping (compile-fails on
  future variants that forget the mapping).

`NodeLinker` gains `Clone + Copy + Eq` so it can be threaded by
value into `Install` and matched in tests.

Add the `--node-linker [isolated|hoisted|pnp]` CLI flag to
`pacquet install`, mirroring pnpm's flag. Overrides
`Config.node_linker` for the invocation; absent flag → config's
yaml/npmrc value wins.

The CLI parses into a `NodeLinkerArg` mirror enum (kept in the
CLI crate so `pacquet-config` stays free of `clap`), then
`into_config()` converts to the canonical
`pacquet_config::NodeLinker`.

Threaded into the `Install` runner as an explicit
`node_linker: NodeLinker` field rather than reading
`config.node_linker` directly. Matches the existing pattern
`supported_architectures` uses (`state.config` is `&'static`, so
the override-merge happens at the CLI layer and lands here as a
fully-resolved value). `build_modules_manifest` consumes it on
the `.modules.yaml.nodeLinker` write so the persisted setting
reflects the invocation, not just the config.

`pacquet add` uses Config's value by default (`add` doesn't
expose `--node-linker` per the umbrella scope; Slice 6's pipeline
integration will revisit if needed).

5 new CLI tests:

- `node_linker_default_is_none` — flag absent → field is None.
- `node_linker_hoisted` / `node_linker_isolated` / `node_linker_pnp`
  — each value parses and round-trips through `into_config()`.
- `node_linker_invalid_value_rejected` — clap rejects unknown
  values with the bad value in the error message.
- `node_linker_arg_into_config_matches_every_variant` — every
  ValueEnum variant has a canonical mapping (compile-fails on
  future variants that forget the mapping).

`NodeLinker` gains `Clone + Copy + Eq` so it can be threaded by
value into `Install` and matched in tests.