Move summary prompt templates to dedicated file

[aseemxs]

Extract large string literals from chat/mod.rs into a new prompts.rs file to improve code organization and maintainability. The functionality remains unchanged while making the code more readable.

Suggested comment in #1128

Description of changes:

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[codecov-commenter]

Codecov Report

Attention: Patch coverage is 0% with 40 lines in your changes missing coverage. Please review.

Project coverage is 13.97%. Comparing base (04b98f4) to head (70b1b21).
Report is 10 commits behind head on main.

Files with missing lines	Patch %	Lines
crates/q_cli/src/cli/chat/prompts.rs	0.00%	38 Missing ⚠️
crates/q_cli/src/cli/chat/mod.rs	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1182      +/-   ##
==========================================
+ Coverage   13.94%   13.97%   +0.02%     
==========================================
  Files        2363     2365       +2     
  Lines      204485   204606     +121     
  Branches   184849   184970     +121     
==========================================
+ Hits        28520    28593      +73     
- Misses     174564   174607      +43     
- Partials     1401     1406       +5     

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