fix: migrate MacPorts paths to Homebrew in agents + scenarios

[bborbe]

What

Sweep remaining MacPorts (/opt/local) paths → Homebrew across the coding plugin's agents and scenarios. MacPorts was removed in the local MacPorts→Homebrew migration; the stale paths no longer resolve.

Changes

• agents/shellcheck-assistant.md — allowed-tools + docs /opt/local/bin/shellcheck → /opt/homebrew/bin/shellcheck; install section drops port install, keeps brew install.
• agents/license-assistant.md — current-year lookup /opt/local/libexec/gnubin/date +%Y → gdate +%Y.
• scenarios/{001-toolchain-preflight,003-scaling-funnel-100-files,004-findings-exist-path}.md — host ast-grep path /opt/local/bin/ast-grep → /opt/homebrew/bin/ast-grep.

Verification

• make precommit — 30/30 pass
• grep -rn "/opt/local" agents scenarios — clean

Docs/config only; no behavior change beyond correcting the tool paths.

[ben-s-pull-request-reviewer]

Based on my review of the PR, here is my assessment:

Summary

The PR correctly migrates MacPorts tool paths to Homebrew equivalents across 5 files:

• shellcheck-assistant.md: /opt/local/bin/shellcheck → /opt/homebrew/bin/shellcheck
• license-assistant.md: MacPorts GNU date path → gdate command
• Three scenario walk files: Updated recorded ast-grep resolution paths to /opt/homebrew/bin/ast-grep

The changes are accurate, consistent, and the CHANGELOG bullet correctly describes all three migration components.

Step 4 early exit: This diff touches only .md files (CHANGELOG.md, agents/*.md, scenarios/*.md). No .go or .py files changed. The agents/** pattern matches, but the changes are path-string migrations with no structural or rule logic changes — no judgment rules are triggered.

{
  "verdict": "approve",
  "summary": "Clean migration of MacPorts tool paths to Homebrew equivalents across shellcheck-assistant, license-assistant, and three scenario walk documents. All path references updated correctly; CHANGELOG bullet accurately describes the scope. No functional logic changes.",
  "comments": [],
  "concerns_addressed": [
    "correctness: MacPorts paths migrated to Homebrew in shellcheck-assistant (allowed-tools + docs)",
    "correctness: GNU date command reference updated to gdate in license-assistant",
    "correctness: ast-grep Homebrew path reflected in scenario 001 walk results",
    "correctness: ast-grep Homebrew path reflected in scenario 003 walk results",
    "correctness: ast-grep Homebrew path reflected in scenario 004 walk results"
  ]
}