Prepare v0.3.1 with interactive mode and coverage improvements

[joelee]

Add continuous interactive prompting with session-aware follow-up flows, improve unit coverage and Codecov reporting, and tighten interactive UX around prompt styling, error recovery, and exit guidance so the new release feels safer and more polished to use.

[codecov]

Codecov Report

❌ Patch coverage is 83.18264% with 93 lines in your changes missing coverage. Please review.
✅ Project coverage is 66.64%. Comparing base (8ee2f37) to head (41b7406).

Files with missing lines	Patch %	Lines
src/lib.rs	78.88%	91 Missing ⚠️
src/shell.rs	93.75%	2 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##             main       #7       +/-   ##
===========================================
+ Coverage   47.89%   66.64%   +18.75%     
===========================================
  Files           8        8               
  Lines        3153     3688      +535     
===========================================
+ Hits         1510     2458      +948     
+ Misses       1643     1230      -413     

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