Harden self-hosted Office smoke execution

[dachent]

Objective

Harden the GitHub Actions trust boundary for self-hosted Office smoke execution. This issue owns CI-runner safety and orchestration only; it does not own the xlsx-win runtime architecture tracked in #33–#39.

Scope

• use a protected GitHub Environment for Office execution
• require explicit approval for fork pull requests and changes to Office harness/runtime code
• separate a trusted base-branch harness from pull-request-produced fixtures and artifacts
• prefer an ephemeral or resettable Windows VM runner
• minimize credentials, mounted data, and outbound network access
• verify test-package hashes before execution
• standardize Word, PowerPoint, and Excel result artifacts
• verify generic Office process cleanup after every run
• preserve safe manual and /office-smoke triggering

Non-goals

• no Excel Path A supervisor or COM adapter design; tracked in xlsx-win v2: implement hardened Path A supervisor and COM adapter #36
• no Path C in-process controller; tracked in xlsx-win v2: build Path C in-process Excel controller #37
• no workbook semantic-validation or publication-gate design; tracked in xlsx-win v2: implement semantic validation, security, and publication gates #38
• no Excel certification corpus, fault injection, or runtime worker-operations architecture; tracked in xlsx-win v2: build certification corpus, fault injection, soak tests, and worker operations #39
• no repository-wide Actions matrix or health-reporting design; tracked in Expand manifest-driven GitHub Actions and repository health reporting #44

Relationship to Excel runtime roadmap

This issue supplies the GitHub Actions execution boundary required by the self-hosted portions of #39. The Excel runtime, worker protocol, certification content, and release gates remain owned by #33–#39.

Acceptance criteria

• pull-request code cannot execute directly on a long-lived Office runner without protected approval
• the trusted harness is sourced from a protected revision
• downloaded test inputs are hash-verified
• runner credentials and local data are isolated from test code
• each Office application emits an independent machine-readable result
• all owned Office processes are cleaned up or explicitly reported
• fork pull requests and harness changes require explicit approval

Follow-up to PR #40.