Fix: Use short git commit ID (7 chars) instead of full hash

Research findings:
- Industry standard uses 'git rev-parse --short HEAD' (7 characters)
- Short hashes are more readable for logs and health endpoints
- Git automatically extends hash if 7 chars aren't unique
- Better for production deployment tracking and version verification

Changes:
- Dockerfile: Use --short flag to get 7-character commit ID
- README: Update examples to show short commit format (0e993d9)
- README: Add explanation of why short commits are used

Example health response:
{
  "git_commit": "0e993d9",  // 7 chars, not full 40-char SHA
  "version": "1.0.0"
}

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

Dockerfile

@@ -16,9 +16,9 @@ RUN npm install --production
 # Copy app code (changes more often — below the cached layer)
 COPY . .
 
-# Try to get git commit if not provided and .git exists
+# Try to get short git commit (7 chars) if not provided and .git exists
 RUN if [ "$GIT_COMMIT" = "unknown" ] && [ -d .git ]; then \
-      GIT_COMMIT=$(git rev-parse HEAD 2>/dev/null || echo "unknown"); \
+      GIT_COMMIT=$(git rev-parse --short HEAD 2>/dev/null || echo "unknown"); \
     fi && \
     echo "export GIT_COMMIT=${GIT_COMMIT}" >> /etc/profile.d/git-version.sh