feat: backends, jj support

[cococonscious]

Implements Jujutsu VCS support by allowing multiple backends to be defined.
When using 'jj describe', previous values are pre-selected/pre-filled to allow for easy editing.
Disclaimer: As I don't use Jujutsu myself, I used Anthropic's Opus 4.6 LLM to assist in the implementation of this feature. Please review carefully.

Refs: #109

[codecov]

Codecov Report

❌ Patch coverage is 89.30481% with 20 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
src/lib/questions.rs	87.50%	11 Missing ⚠️
src/lib/vcs.rs	90.81%	9 Missing ⚠️

Files with missing lines	Coverage Δ
src/lib/commit.rs	100.00% <ø> (ø)
src/lib/config.rs	100.00% <100.00%> (ø)
src/lib/vcs.rs	90.81% <90.81%> (ø)
src/lib/questions.rs	92.30% <87.50%> (-5.03%)	⬇️

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