refactor(hm-dsl-engine): Replace stringly-typed run() mode ("list"/"render") with an internal enum

[markovejnovic]

The smell

SubprocessTsEngine::run took the operation as a raw mode: &str, and the two
call sites passed the string literals "list" and "render". The same magic
strings were also branched on inside the embedded JS runner. The variant set was
implicit, and a typo like "lst" would compile fine and only fail at runtime in
the spawned JS process.

The change

Introduced a small private enum RunMode { List, Render } with an
as_arg(self) -> &'static str conversion. run now takes RunMode instead of
&str; list_pipelines passes RunMode::List and render_pipeline_json
passes RunMode::Render. The wire string now appears in exactly one place — the
enum's as_arg match.

This is fully crate-internal (a private method on a private enum), so the blast
radius is zero and the change is behavior-preserving: the exact same "list" /
"render" argument is still passed to the JS runner.

Type-safety pattern

Follows the fd "ValueEnum / strategy-enum over stringly-typed flag" pattern: a
closed set of operations modeled as an enum makes the dispatch exhaustive and
typo-proof at compile time, while keeping a single source of truth for the wire
string.

Validation

Only cargo check -p hm-dsl-engine was run locally (passes). Full CI runs on
this PR.