Refactor: remove aicpu_build_graph runtime

[ChaoWao]

Summary

All aicpu_build_graph features have been merged into
tensormap_and_ringbuffer, so the standalone runtime is no longer
needed. This change removes it entirely.

• Delete src/a2a3/runtime/aicpu_build_graph/ and the corresponding ST
  test tree under tests/st/a2a3/aicpu_build_graph/
• Drop the test_discovers_aicpu_build_graph discovery test and the
  matching skip clause in tests/conftest.py
• Drop ABG_EXAMPLE_CASES and the runtime branch from
  tools/benchmark_rounds.sh; redirect the verify_packaging.sh smoke
  test to the tensormap_and_ringbuffer paged_attention_unroll case
• Remove the runtime from issue templates, .claude rules / skills /
  commands, READMEs, the per-arch runtime docs, the tensor-dump and
  dynamic-linking docs, and the L2 perf header comments

Additional cleanups

• Fix stale verification-matrix copy: tools/README.md,
  tools/verify_packaging.sh, and docs/python-packaging.md all said
  "5 install paths × 4 entry points" but only two user-facing entry
  points exist (pytest, standalone test_*.py); update to "× 2"
• Update tools/README.md to refer to the single TMR_EXAMPLE_CASES
  map in benchmark_rounds.sh (was "EXAMPLE_CASES maps … per runtime"
  back when ABG had its own map)
• Widen the scene-test retry from rc==124 only to any non-zero rc
  in all four spots (st-sim-a2a3, st-sim-a5, st-onboard-a2a3,
  st-onboard-a5), so transient PTO-ISA git-clone failures (e.g.
  SSL_ERROR_SYSCALL) trigger the pinned-commit retry instead of
  failing the job outright

Testing

• Simulation tests pass
• Hardware tests pass

[gemini-code-assist]

Code Review

This pull request removes the aicpu_build_graph runtime variant from the repository, leaving host_build_graph and tensormap_and_ringbuffer as the remaining supported runtimes. The deletion includes the runtime's source code, headers, build configurations, and all associated examples and tests. Documentation, issue templates, and various tools and scripts have been updated to reflect this architectural simplification. I have no feedback to provide.