feat: add shared git hooks with setup script

Git hooks were only in .git/hooks/ which isn't tracked by git.
Created a shared githooks/ directory so all contributors get
the pre-commit hook that enforces SDLC requirements.

Changes:
- githooks/pre-commit: Pre-commit hook enforcing SDLC (lint, tests, docs)
- scripts/setup-githooks.sh: Script to configure git core.hooksPath
- README.md: Added first-time setup instructions
- AGENTS.md: Updated startup steps and project structure

Users/agents run ./scripts/setup-githooks.sh after cloning to enable hooks.

Reference: docs/SDLC.md

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Assisted-by: GLM-5 via Crush <crush@charm.land>

AGENTS.md

@@ -119,6 +119,8 @@ A pre-commit hook automatically enforces SDLC requirements:
     └── security-model.md
 
 src/                       # Source scripts
+scripts/                   # Utility scripts (setup-githooks.sh)
+githooks/                  # Shared git hooks (pre-commit)
 config/                    # Configuration files
 ├── includes.installer/    # Installer configs (preseed.cfg)
 ├── hooks/live/            # Live system hooks
@@ -135,6 +137,9 @@ output/                    # Build artifacts
 
 ### 1. Start Up
 ```bash
+# Configure git hooks (if not already done)
+./scripts/setup-githooks.sh
+
 # Check current state
 ls -lh output/
 git log --oneline -10