feat: interactive --generate mode with editor and re-generation

[suenot]

Description

Enhance commit message generation with an interactive flow:

1. Generate message and show it
2. Ask: [a]ccept / [e]dit in $EDITOR / [r]egenerate / [c]ontext + regenerate
3. If context: prompt user to type extra context, then regenerate with it
4. Loop until accepted

This is different from --dry-run (which just previews) — this leads to an actual commit.

Example

aicommit --generate
# Generated: "fix: handle nil pointer in auth"
# [a]ccept  [e]dit  [r]egenerate  [c]ontext? a