refactor: rename cmcp_gateway -> cmcp_runtime

[imran-siddique]

Summary

• Renames src/cmcp_gateway/ to src/cmcp_runtime/ via git mv (history preserved, similarity scores 88-100%)
• Updates all import paths across src/, tests/, and pyproject.toml
• Symbol renames: GatewayClaim -> RuntimeClaim, GatewayContext -> RuntimeContext, GatewayError -> CMCPError (avoids Python builtin conflict), gateway_url -> runtime_url
• pyproject.toml: package name cmcp-gateway -> cmcp-runtime, entry point, packages, mypy files and overrides
• _GATEWAY_VERSION -> _RUNTIME_VERSION in trace_claim.py and corresponding test
• "cmcp-gateway" PyPI lookup string annotated with # was cmcp-gateway; SPIFFE URI spiffe://cmcp.gateway/... left unchanged (protocol identifier)
• MCPGateway and CMCPProxy left unchanged per spec

Not changed

• cmcp_verify package name
• Markdown files (handled separately)
• cmcp-gateway references in CHANGELOG.md (historical)
• Badge URLs and shield.io links

Test results

• ruff check src/ tests/: all checks passed (1 auto-fix applied)
• mypy src/cmcp_runtime/: no issues in 34 source files
• pytest tests/unit/ (excluding test_benchmarks.py): 582 passed, 1 skipped
• test_benchmarks.py::test_benchmark_runs_smoke is a pre-existing failure on main (Cedar policy deny in benchmark setup, unrelated to this rename - confirmed by checking origin/main directly)

Test plan

• CI passes on all unit test jobs
• Confirm test_benchmarks.py failure exists on main before merge (pre-existing)
• Verify pip install -e . resolves the package as cmcp-runtime
• Verify cmcp entrypoint still works after install