feat: implement immediate execution plan foundations

[joshellis625]

Summary

• add workflow foundation: CI, PR template, CONTRIBUTING standards
• consolidate docs into canonical PROJECT_GUIDE + workflow/playbook docs
• archive historical markdown snapshots and repoint legacy docs
• apply UI/UX pass A for message input, message bubbles, and chat toolbar
• add Cmd+N shortcut for new chat
• add targeted tests for chat override behavior and totals
• add local CI-mirroring script

Testing Evidence

• CI workflow added for macOS build + test
• Local CLI in Codex sandbox cannot fully execute SwiftData macro plugins in this environment; verify in Xcode and GitHub Actions

Rollback

• revert commit 19aa363 to restore previous behavior and docs layout