fix(cli): smart paste — 6 review fixes from PR #4764 feedback

Six bugs identified during PR #4764 review, applied as an atomic commit:

1. DRY _make_paste_marker() helper — all 3 call sites (handle_paste,
   _on_text_changed, _reconstruct_pastes) now share one canonical function,
   eliminating the char vs line format mismatch that broke paste reconstruction.

2. MAX_PASTE_SIZE = 100_000 memory cap — pastes > 100 KB are inserted as
   plain text to prevent OOM from hoarding oversized payloads.

3. Deletion loop: remove early return — replaced with any_deleted flag so
   every active paste_id is checked per keystroke; one Delete can now clear
   multiple stale markers instead of stopping at the first match.

4. Fallback: store pasted_portion not full buffer — _on_text_changed now
   saves text[prev_len:] (the newly pasted chunk) rather than buf.text in
   its entirety, so pre-existing typed content is never hoisted into the
   paste store.

5. Unified _PASTE_MARKER_RE regex — _reconstruct_pastes uses a single regex
   matching both [+X line(s)] and [+X char(s)] formats, making it robust
   against any inconsistency between the bracketed-paste path and the
   fallback detection path.

6. Threshold aligned to 20 chars — both handle_paste and _on_text_changed
   now use char_count >= 20 as the collapse trigger, consistent with the
   documented contract.

Author: Stefan