Add 6 software development skills

- code-review: critical code review of uncommitted changes
- code-self-review: self-review after writing code
- code-cleanup: garbage collection for technical debt
- security-review: security review from pentester perspective
- create-merge-request: create GitLab MR from uncommitted changes
- understand-project: learn about a project from docs and structure

code-self-review/SKILL.md

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+---
+name: code-self-review
+description: >
+  Review your own work for improvements. Step back and critically evaluate code you just
+  wrote, checking for simplicity, dead code, cleanup opportunities, and missing pieces.
+---
+
+# Self-Review Your Work
+
+Now that you have written the code, take a step back and critically evaluate your own work. Pretend you're reviewing someone else's code with fresh eyes.
+
+## Step 1: Review What You Wrote
+
+Go through all the changes you made in this session:
+- Run `git diff` to see the uncommitted changes
+- Re-read the code you wrote or modified
+- Look at each file you touched
+
+## Step 2: Ask Yourself These Questions
+
+### Could this be simpler?
+- Is there a more straightforward way to achieve the same result?
+- Did you over-engineer or add unnecessary abstraction?
+- Are there complex patterns that could be replaced with simpler ones?
+- Could any functions be shorter or more focused?
+
+### Is there code that's no longer needed?
+- Did you write helper functions that ended up unused?
+- Are there variables, imports, or constants that aren't being used?
+- Did you leave any debug statements, console logs, or commented code?
+- Did earlier iterations leave behind dead code?
+
+### Can anything be cleaned up?
+- Are there inconsistent naming conventions?
+- Is there duplicated code that could be consolidated?
+- Are there magic numbers or strings that should be constants?
+- Could the code be better organized or structured?
+
+### Is anything missing?
+- Look back at the original task or implementation plan
+- Did you implement everything that was requested?
+- Are there edge cases you discussed but didn't handle?
+- Did you skip any error handling or validation?
+- Are there TODOs you left that should be addressed now?
+
+## Step 3: Report Your Findings
+
+Provide an honest assessment:
+
+### Things I Would Improve
+List specific changes you'd make, with file names and descriptions.
+
+### Code to Remove
+Any unnecessary code you spotted that should be deleted.
+
+### Missing Pieces
+Anything from the original plan that still needs to be implemented.
+
+### Overall Assessment
+Is the code in a good state, or does it need more work before it's ready?
+
+---
+
+After this review, ask if the user wants you to make any of the identified improvements.