SUBS-014: misleading "does not exist — skipped" for a plain untracked dir

Now: DONE

LOW (UX — found mid-fix in SUBS-009). be put <dir> where <dir> is a plain on-disk untracked directory named without a trailing slash returns put: <dir> does not exist — skipped / PUTNONE — misleading, because the dir is right there. The cause is the same SNIFFClassify file-iteration gap SUBS-009 fixed for the sub-add case: a bare dir arg is never matched (only <dir>/ dir-form is). This is not sub-specific — it fires for any ordinary untracked dir.

Repro

mkdir newdir && touch newdir/a.c (untracked).
be put newdir → does not exist — skipped / PUTNONE.
be put newdir/ (trailing slash) → stages the subtree correctly.

Expected

Either stage the dir (treat a bare untracked dir like <dir>/), or refuse with a "did you mean newdir/?" hint — never the false "does not exist".

Fix sketch

In sniff/PUT.c PUTStage, where SUBS-009 added the sub-add candidate branch, add a sibling branch for a plain untracked dir that emits the trailing-slash hint instead of the generic skip. Decide stage-vs-hint.

Tests

A test/put case once the behavior (auto-stage vs hint) is decided.