refactor(hm-exec): Replace raw-string-literal job-state match with a typed classifier

[markovejnovic]

The smell

The cloud watch loop decides whether a job's logs exist by round-tripping the
typed job state through to_string() and matching the result against a
hand-written set of string literals (crates/hm-exec/src/cloud/watch.rs:113):

let state = job.state.to_string();
let logs_available = matches!(
    state.as_str(),
    "running" | "passed" | "failed" | "timed_out"
        | "canceling" | "canceled" | "timing_out"
);

This is stringly-typed flag matching. A typo in any literal compiles fine and
silently drops a job's logs, and there is no exhaustiveness check: when the
cloud adds a new JobState, this matches! keeps compiling and silently treats
the new state as "no logs".

The change

harmont_cloud already exposes job.state as a typed, exhaustive
JobState enum (it only Displays into those strings). This PR matches on the
enum variants directly via a single named, documented predicate
job_logs_available(state: JobState) -> bool, placed alongside the file's other
helpers and unit-tested. The string round-trip (to_string()/as_str()) is
removed at this call site. Behavior is preserved exactly: the true set is
Running | Passed | Failed | TimedOut | Canceling | Canceled | TimingOut, and
Pending | Scheduled | Assigned | Skipped are false (matching the original
literal set — skipped was deliberately excluded before and remains so).

Because the predicate matches every JobState variant explicitly (no
wildcard arm), adding a new cloud state becomes a compiler-visible change rather
than a silent miss.

Type-safety pattern

This applies the ValueEnum / typed-classifier doctrine seen in tools like
ripgrep and fd: push known finite string sets behind a typed enum match so
typos can't compile and missing cases are caught by exhaustiveness checking,
instead of duplicating literal sets at each call site.

Testing note

Only cargo check -p hm-exec was run locally (per task scope; the full
workspace/TS build was not run). Note: the local sandbox has a pre-existing,
unrelated compile error in crates/hm-exec/src/local/backend.rs
(ImageRegistry::open now expects NonZero<u64>, a hm-vm dependency skew)
that reproduces on the base branch and is independent of this change — the
compiler reached and resolved watch.rs with no diagnostics. Full CI runs on
this PR. Draft = needs review.