Refactor test scripts into modular examples directory with dual-engine driver architecture

[smilingday]

This PR comprehensively refactors the legacy test_scripts/ directory into a highly modular, reusable examples/ directory featuring a dual-engine driver architecture.

Key Improvements

1. Directory & File Normalization:

   • Renamed test_scripts/ to examples/.
   • Flattened utility locations: Moved models/utils.sh and models/utils.ps1 to examples/utils.sh and examples/utils.ps1.

2. Modular Command-Specific Test Suite (examples/commands/):

   • Replaced the monolithic, unmaintainable litert_cli_test_commands.sh with 5 pristine, independent test scripts: download_test.sh, compile_test.sh, run_test.sh, benchmark_test.sh, and visualize_test.sh.
   • Stripped out all verbose $TEST_DATA_DIR and $MODEL_DIR variables in favor of clean symlinked workspaces.

3. Dual-Engine Driver Architecture:

   • examples/run_commands.sh: Automated regression driver for all CLI commands.
   • examples/run_models.sh: Automated driver for model-specific end-to-end demos.
   • Both drivers support comma-separated execution lists (download,compile) and default to running all tests when invoked without arguments.
   • Integrated LITERT_CLI_SHARED_VENV="true" caching mode, drastically reducing CI execution time by sharing the UV environment and model caches across all test suites.

4. Platform & Hardware Robustness:

   • Added standard Linux host checks around offline AOT compilation steps across all scripts, elegantly skipping them on macOS to prevent CI failures.
   • Documented and commented out Android OpenCL GPU runs for ResNet18 due to known PADV2 op delegation limitations.

5. Comprehensive Documentation Sync:

   • Updated README.md, pyproject.toml, and SKILL.md to reflect the new examples/ structure and execution SOPs.