ci: add trusted ephemeral Ascend runner

[ShuhaoZhangTony]

What changed

• add a main-only manual Ascend host regression workflow with read-only permissions
• require a selected-workflow organization runner group, main-only environment, and one-job ephemeral ARM64 runner
• add non-destructive NPU/device-node, npu-smi, Docker binding, graph-mode, app-health, cleanup, and report checks
• add an operator wrapper that downloads and verifies the latest runner, registers it once, dispatches the job, watches it, and verifies deregistration
• document the public-repository trust boundary and release v4.6.31

Security boundary

Normal pull requests remain on GitHub-hosted runners. The NPU host workflow has no pull_request or push trigger, checks the exact trusted main SHA, uses a workflow-restricted group and main-only environment, and never starts/replaces the managed model service.

Validation

• workflow YAML and shell syntax passed
• Ruff passed
• focused deployment/security tests: 42 passed
• full suite: 726 passed, 28 skipped
• direct non-destructive host probe: 8 NPU nodes, control devices, npu-smi, container binding, graph contract, app v4.6.30 health all passed
• failure artifact path verified

Closes #8