chore: bump Formula sha256 to v0.5.0

Written by Minimax-M2.7 in Claude Code Harness.

Replaces the placeholder with the real SHA256 of the v0.5.0 tarball:
6bec75321979828243fbc437843393d85f068affaed8c3370f0531c6086c19bd

Computed from the GitHub-generated archive. `brew update && brew
upgrade agentic-stack` will now fetch the correct artifact.

Author: codejunkie99

Formula/agentic-stack.rb

@@ -2,7 +2,7 @@ class AgenticStack < Formula
   desc "One brain, many harnesses — portable .agent/ folder for AI coding agents"
   homepage "https://github.com/codejunkie99/agentic-stack"
   url "https://github.com/codejunkie99/agentic-stack/archive/refs/tags/v0.5.0.tar.gz"
-  sha256 "0000000000000000000000000000000000000000000000000000000000000000"
+  sha256 "6bec75321979828243fbc437843393d85f068affaed8c3370f0531c6086c19bd"
   version "0.5.0"
   license "MIT"