feat(find): fall back to native find for unsupported predicates

[hgunduzoglu]

Summary

Fixes #2469. rtk find refused compound predicates and actions — -not, !, -or/-o, -and/-a, -exec, -size, -mtime, -regex, etc. — by printing rtk find does not support compound predicates or actions … Use \find` directly.` and producing no results. That silently breaks the user's command, which violates RTK's Never-Block principle ("if a filter fails, fall back to raw output; RTK should never prevent a command from executing").

Change

When rtk find sees a predicate it can't compact, it now falls back to native find (unfiltered passthrough) instead of refusing, and propagates native find's exit code. The common -name / -type / -maxdepth path is untouched and keeps full rtk compaction.

# before — command refused, no results
$ rtk find . -not -name "*.txt"
rtk find does not support compound predicates or actions (e.g. -not, -exec). Use `find` directly.

# after — native find runs, results returned
$ rtk find . -not -name "*.txt"
.
./keep.log
./a
./a/x.rs
./b

This also makes -exec, -o, -size, -mtime, … work for the first time, since they all reach native find now.

Notes

• Token savings: the fallback path is intentionally unfiltered (same contract as rtk proxy) — a correct full result beats a broken/refused command. The compactable majority (-name/-type/-maxdepth) still gets rtk's tree formatting. Full compaction of compound-predicate output could be a later enhancement; this PR is the Never-Block fix.
• parse_find_args keeps its bail as a defensive contract (its unit tests still pass); run_from_args now intercepts unsupported flags before parsing and routes to passthrough.
• run_from_args now returns Result<i32> to propagate the child exit code; the main.rs Find arm returns it.

Testing

• 4 new tests: -not passthrough returns Ok(0), -exec passthrough returns Ok(0), has_unsupported_find_flags detects unsupported / ignores supported predicates.
• Existing parse_find_args bail tests and run_from_args compaction tests still pass.
• cargo fmt --all --check && cargo clippy --all-targets && cargo test — clean, 2202 passed.
• Manual verification vs system find for -not, -o, -exec, and exit-code propagation on an invalid query (rtk exit 1 == native exit 1).

[hgunduzoglu]

Fixed the Windows CI failure in 352e7b2. The two passthrough tests executed the resolved find and asserted GNU/BSD semantics (valid -not/-exec query exits 0), but on Windows find.exe is an unrelated text-search tool that rejects those args (exit 2). Gated both behind #[cfg(unix)]; the routing decision stays covered cross-platform by has_unsupported_find_flags_detects_and_ignores.