feat(tui): make paste summary thresholds configurable

[aspiers]

Closes #14712

What this does

Adds two optional fields to the experimental config block:

• paste_min_lines — minimum line count before pasted content is summarised (default: 3)
• paste_min_length — minimum character length before pasted content is summarised (default: 150)

Both default to the current hardcoded values, so existing behaviour is fully preserved for anyone who doesn't set them.

Why

The current thresholds are too aggressive for users of voice dictation software (e.g. Whisper Flow, Whispering). A single dictated utterance frequently exceeds 150 characters, causing it to be immediately collapsed into [Pasted ~1 lines] — making it impossible to spot and correct small transcription errors. The existing disable_paste_summary flag is a blunt workaround that sacrifices the feature entirely, which is painful when you then paste a large file or stack trace.

Configuration example

{
  "experimental": {
    "paste_min_lines": 6,
    "paste_min_length": 500
  }
}

Notes

• Flat key naming is consistent with existing experimental config style
• No new tests added — no other experimental config fields have tests
• Prior art: PR feat: add configurable paste summary thresholds #1960 attempted this with a nested experimental.editor.summary_thresholds structure; this uses flat keys for consistency

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