Draft: feat: add a skill command

[GG-HH]

This command allow one to add a default skill through ggshield

Context

What has been done

Validation

PR check list

• As much as possible, the changes include tests (unit and/or functional)
• If the changes affect the end user (new feature, behavior change, bug fix) then the PR has a changelog entry (see doc/dev/getting-started.md). If the changes do not affect the end user, then the skip-changelog label has been added to the PR.

[codecov]

Codecov Report

❌ Patch coverage is 97.40260% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 92.66%. Comparing base (6b08b22) to head (fb5f1df).
⚠️ Report is 5 commits behind head on main.

Files with missing lines	Patch %	Lines
ggshield/cmd/skill/uninstall.py	89.47%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1188      +/-   ##
==========================================
+ Coverage   92.61%   92.66%   +0.04%     
==========================================
  Files         160      165       +5     
  Lines        7731     7808      +77     
==========================================
+ Hits         7160     7235      +75     
- Misses        571      573       +2     

Flag	Coverage Δ
unittests	92.66% <97.40%> (+0.04%)	⬆️

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[paulpetit-gg-ext]

(I know this is still marked as draft ^^)

Since we are planning to add other AI-related commands like MCP configuration discovery, I wonder if we should create a more general ai vertical?

Same, the hook to prevent secrets from being sent to LLMs (#1183) is currently under ggshield install and ggshield secret scan ai-hook but maybe we could move it under this new vertical.

Otherwise, other idea: have this command under ggshield install instead. 🙃

What do you think?

[GG-HH]

That's actually a good remark, I agree that we should not go in all directions and it can probably be a sub-element of ggshield install group. I would not use a generic 'ai' path because i believe we will add AI interactions in almost all commands at some point (I would differentiate: how ggshield secures your AI workloads and how can you use ggshield with AI).
TBH, this MR is not meant to be merged, I wanted it to support a few discussions and I am very happy to get your comment!