feat: Add shellcheck CI for bash scripts

[michael-wojcik]

Summary

Add shellcheck static analysis to CI for all bash scripts in the repository.

Context

The repository has several verification scripts (verify-*.sh). Shellcheck would catch common bash issues:

• Unquoted variables
• POSIX compatibility
• Common pitfalls

Proposed Implementation

1. Create .github/workflows/shellcheck.yml
2. Run shellcheck on all .sh files
3. Fail CI on shellcheck warnings

Prerequisites

• CI infrastructure needs to be set up first (no .github/workflows/ exists currently)

Related

• Phase E Worktree Integration (PR feat: Worktree Integration (Phase E) #144)
• verify-worktree-integration.sh