🎨 Palette: Route logs to stderr, keeping data on stdout

[ffalcinelli]

💡 What: Changed all instances of UI logs/prompts across the project to output to stderr (using eprintln! instead of println!), specifically targeting standard logs (info, error, success, warn) in src/utils/ui.rs and across all action scripts.
Crucially, the diff outputs inside plan logic (like print_diff) remain on stdout.
🎯 Why: Standard Unix philosophy dictates that informational/interactive logs should go to stderr, while the actual output payload goes to stdout. This allows users to safely pipe kaji commands (e.g. kaji plan > changes.patch) without UI loading indicators, checkmarks, or warnings polluting the redirected file.
📸 Before/After:
Before:

$ kaji plan > patch
# The patch file now contains "✅ No changes planned" and diff headers.

After:

$ kaji plan > patch
✅ No changes planned # Prints to terminal correctly
# The patch file is perfectly clean or empty

♿ Ergonomics: Vastly improves scripting, composition, and log redirection.

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PR created automatically by Jules for task 5503033966193379511 started by @ffalcinelli

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[codecov]

Codecov Report

❌ Patch coverage is 97.87234% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 96.13%. Comparing base (35371f9) to head (c707558).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/inspect.rs	80.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #175      +/-   ##
==========================================
+ Coverage   96.10%   96.13%   +0.03%     
==========================================
  Files          29       29              
  Lines        2898     2926      +28     
==========================================
+ Hits         2785     2813      +28     
  Misses        113      113              

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