feat(agents): добавлены скиллы для ревью, GitHub, диагностики и менторинга

- Зачем:
  - расширить набор переиспользуемых скиллов для повседневных задач: ревью PR, работа с GitHub, проверка dotfiles, диагностика сети, ревью кода менти.
- Что:
  - добавлен differential-review (Trail of Bits) — security-focused ревью PR.
  - добавлен github (Callstack) — workflow через gh CLI, stacked PRs.
  - создан dotfiles-check — bash-скрипт проверки целостности stow-симлинков.
  - создан proxy-debug — диагностика сети и прокси hproxy.local.
  - создан review-homework — ревью кода менти в обучающем формате.
- Проверка:
  - bash ~/.agents/skills/dotfiles-check/check.sh ~/dotfiles

agents/.agents/skills/github/SKILL.md

@@ -0,0 +1,60 @@
+---
+name: github
+description: GitHub patterns using gh CLI for pull requests, stacked PRs, code review, branching strategies, and repository automation. Use when working with GitHub PRs, merging strategies, or repository management tasks.
+license: MIT
+metadata:
+  author: Callstack
+  tags: github, gh-cli, pull-request, stacked-pr, squash, rebase
+---
+
+# GitHub Patterns
+
+## Tools
+
+Use `gh` CLI for all GitHub operations. Prefer CLI over GitHub MCP servers for lower context usage.
+
+## Quick Commands
+
+```bash
+# Create a PR from the current branch
+gh pr create --title "feat: add feature" --body "Description"
+
+# Squash-merge a PR
+gh pr merge <PR_NUMBER> --squash --title "feat: add feature (#<PR_NUMBER>)"
+
+# View PR status and checks
+gh pr status
+gh pr checks <PR_NUMBER>
+```
+
+## Stacked PR Workflow Summary
+
+When merging a chain of stacked PRs (each targeting the previous branch):
+
+1. **Merge the first PR** into main via squash merge
+2. **For each subsequent PR**: rebase onto main, update base to main, then squash merge
+3. **On conflicts**: stop and ask the user to resolve manually
+
+```bash
+# Rebase next PR's branch onto main, excluding already-merged commits
+git rebase --onto origin/main <old-base-branch> <next-branch>
+git push --force-with-lease origin <next-branch>
+gh pr edit <N> --base main
+gh pr merge <N> --squash --title "<PR title> (#N)"
+```
+
+See [stacked-pr-workflow.md][stacked-pr-workflow] for full step-by-step details.
+
+## Quick Reference
+
+| File | Description |
+| --- | --- |
+| [stacked-pr-workflow.md][stacked-pr-workflow] | Merge stacked PRs into main as individual squash commits |
+
+## Problem -> Skill Mapping
+
+| Problem | Start With |
+| --- | --- |
+| Merge stacked PRs cleanly | [stacked-pr-workflow.md][stacked-pr-workflow] |
+
+[stacked-pr-workflow]: references/stacked-pr-workflow.md