Add a bracketed paste transformer hook

[M09Ic]

Background

Interactive AI and command consoles often need to handle large multiline pastes differently from normal typing. A common UX is the Claude Code-style paste reference: the editable prompt shows a compact placeholder such as [Pasted text #1 +6 lines], while the application keeps the original pasted payload and expands it later when executing or sending the request.

Today readline normalizes a bracketed paste and inserts it directly into the line buffer. Applications that want the placeholder/reference behavior have to duplicate paste handling or patch readline internals.

Approach

• Add Shell.PasteTransformer and SetPasteTransformer.
• Route bracketed paste insertion through a helper that preserves the existing CRLF/CR normalization.
• Let applications rewrite the pasted payload before insertion, including replacing it with a placeholder like [Pasted text #1 +6 lines].
• Treat an empty transformed string as "do not insert".

The reference store and later expansion are intentionally left to the application layer. Readline only provides the safe hook at the point where terminal paste has already been read and normalized.

Tests

• Added coverage that the transformer receives normalized text.
• Added coverage that a transformer can rewrite or drop pasted text.
• Ran go test ./...

[codecov]

Codecov Report

❌ Patch coverage is 84.61538% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 38.37%. Comparing base (088046b) to head (77de7c1).

Files with missing lines	Patch %	Lines
shell.go	50.00%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #113      +/-   ##
==========================================
+ Coverage   38.33%   38.37%   +0.03%     
==========================================
  Files          59       59              
  Lines        9516     9525       +9     
==========================================
+ Hits         3648     3655       +7     
- Misses       5785     5786       +1     
- Partials       83       84       +1     

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