Add example projects covering all OpenCAD API surfaces

[Copilot]

Users had no runnable reference material to learn from or use as a starting point for building on top of OpenCAD's APIs. Adds an examples/ directory with six self-contained projects spanning every major API layer.

Examples

• 01_hello_part/ — headless fluent API: box + cylinder + boolean cut + fillet + STEP export
• 02_parametric_bracket/ — parameterised sketch → extrude → linear pattern → fillet; tunable constants at the top of the file
• 03_rest_api_client/ — kernel REST API via stdlib urllib: health checks, list ops, create shapes, mesh fetch, topology, replay
• 04_sketch_solver/ — solver REST API: solve, check, and diagnose (DOF, Jacobian sparsity, over/under-constrained variables)
• 05_feature_tree/ — tree REST API: CRUD, rebuild with stale propagation, suppression, branching, and serialize/deserialize round-trip
• 06_agent_chat/ — agent REST API: natural-language design prompts, multi-turn conversation, reasoning mode

Each example has a README.md describing prerequisites and expected output. Examples 01–02 run in-process with no services; 03–06 require the relevant backend services.

Tests

examples/tests/test_examples.py provides 7 pytest tests that import and execute the headless examples end-to-end, asserting on tree node count, operation set, node statuses, and that design parameters (pattern count, fillet radius) round-trip correctly.

# Example: run the bracket script and assert its feature DAG
reset_default_context()
mod = _load_example("02_parametric_bracket/bracket.py")
mod.main()

ctx = get_default_context()
operations = {n.operation for n in ctx.tree.nodes.values()}
assert "create_sketch" in operations
assert "linear_pattern" in operations
assert pattern_nodes[0].parameters["count"] == mod.HOLE_COUNT

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