fix(hat): don't rename to a stem-less dotfile on a degenerate model answer

[marksverdhei]

Bug

sanitize_name appends the original extension unconditionally:

if preserve_ext == "true" and orig_ext:
    if "." in name:
        name = name.rsplit(".", 1)[0]
    name = name + "." + orig_ext

So when the model returns nothing usable — an empty answer, a think-only answer (a reasoning model that spent its budget inside <think> and emitted no name), or punctuation like . / / — the result is a stem-less name like .jpg:

sanitize_name ""                              "true" "jpg"  ->  .jpg
sanitize_name "<think>…ran out of tokens</think>" "true" "jpg"  ->  .jpg
sanitize_name "."                             "true" "jpg"  ->  .jpg

That's non-empty, so it slips past the caller's [[ -z "$suggested" ]] guard (hat:991) and renames the user's file to a hidden, stem-less file — effectively losing it.

Fix

Strip leftover dots/dashes from the stem; if nothing remains, emit no name so the empty-guard rejects the rename and reports (no suggestion):

    name = name.strip("-.")
    name = name + "." + orig_ext if name else ""

Tests

3 regression tests (empty / think-only / punctuation-only → ""). They fail on the old code (actual: .jpg) and pass with the fix. All existing sanitize cases unchanged. 80/80 bats pass.

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