Update external-dependencies README with air-gapped environment documentation

- Added section on offline cache structure and usage
- Documented download scripts (download-all-dependencies.sh, download-deb-packages.sh, verify-offline-cache.sh)
- Added transfer procedure for air-gapped environment
- Referenced offline-cache/README.md for detailed instructions

Co-authored-by: Copilot <2235566219+Copilot@users.noreply.github.com>
Entire-Checkpoint: 01M6PG0QFA4ZGZZA6Y469Q591R

external-dependencies/README.md

@@ -80,6 +80,59 @@ This generates:
 - `<package>-provenance.json`: SLSA v1.0 provenance
 - `<package>-licenses.txt`: License compliance report
 
+## Air-Gapped Environment Support
+
+For air-gapped deployment, offline dependency packages are prepared and transferred via secure media:
+
+### Offline Cache Structure
+
+```
+external-dependencies/offline-cache/
+├── qt6/6.10.2/gcc_64/          # Qt 6.10.2 installation (2.2GB)
+├── src/                        # Source tarballs
+│   ├── KDReports-2.3.0.tar.gz
+│   ├── Log4Qt-2.0.0.tar.gz
+│   └── grantlee-*.tar.gz
+├── deb/                        # Ubuntu .deb packages (49 packages, 51MB)
+└── scripts/                    # Installation scripts
+    ├── install-offline.sh      # Offline installation script
+    └── README.md               # Package inventory
+```
+
+### Download Scripts
+
+**download-all-dependencies.sh**: Downloads all dependencies for offline use
+```bash
+cd external-dependencies/scripts
+./download-all-dependencies.sh [output-directory]
+```
+
+**download-deb-packages.sh**: Downloads Ubuntu .deb packages
+```bash
+./download-deb-packages.sh [output-directory]
+```
+
+**verify-offline-cache.sh**: Verifies offline cache completeness
+```bash
+./verify-offline-cache.sh
+```
+
+### Transfer to Air-Gapped Environment
+
+1. Run download scripts on internet-connected machine
+2. Copy `offline-cache/` directory to secure media (encrypted USB drive)
+3. Transfer to air-gapped environment
+4. Mount or copy to Jenkins build server
+5. Use offline installation scripts in Dockerfiles
+
+### Important Notes
+
+- **Offline cache is excluded from git** (2.3GB, security best practices)
+- Transfer via secure media only
+- Verify checksums after transfer
+- Keep offline cache updated with dependency version changes
+- See `offline-cache/README.md` for detailed usage
+
 ## Version Pinning Strategy
 
 All dependencies use **exact version pinning**: