[AI discovered CI] Signing fails on Unix legs when sn.exe exits with Exec format error

## Build Information
- Current build: https://dev.azure.com/dnceng-public/cbb18261-c48f-4abb-8651-8cdcb5474649/_build/results?buildId=1587866
- Branch: `refs/heads/release/8.0.1xx`
- Failing task: `Build` in `signing` phase; failure type: `tool-execution-error`
- Evidence sources: `azure-timeline`, `azure-task-log`
- **Failure fingerprint:** `signing|tool-execution-error|build|.packages-microsoft.dotnet.arcade.sdk-..-beta..-tools-sign.proj---error-msb4018-the-microsoft.dotnet.signtool.signtooltask-task-failed-unexpectedly.-system.com`
- Matching builds: [1583437](https://dev.azure.com/dnceng-public/cbb18261-c48f-4abb-8651-8cdcb5474649/_build/results?buildId=1583437), [1583291](https://dev.azure.com/dnceng-public/cbb18261-c48f-4abb-8651-8cdcb5474649/_build/results?buildId=1583291), [1579625](https://dev.azure.com/dnceng-public/cbb18261-c48f-4abb-8651-8cdcb5474649/_build/results?buildId=1579625), [1579289](https://dev.azure.com/dnceng-public/cbb18261-c48f-4abb-8651-8cdcb5474649/_build/results?buildId=1579289), [1558381](https://dev.azure.com/dnceng-public/cbb18261-c48f-4abb-8651-8cdcb5474649/_build/results?buildId=1558381)

## Failure History
The same stable signing mechanism appears in the current build and five related builds on independent commits. All supplied occurrences are failures; no intervening pass is provided, so onset and pass/fail boundaries are unknown. Package versions vary, but the attempted executable and `Exec format error` remain stable.

## Error Details
Exact excerpt: `.packages/microsoft.dotnet.arcade.sdk/8.0.0-beta.26457.4/tools/Sign.proj(59,5): error MSB4018: The "Microsoft.DotNet.SignTool.SignToolTask" task failed unexpectedly. System.ComponentModel.Win32Exception (8): An error occurred trying to start process '/.packages/sn/1.0.0/sn.exe' ... Exec format error`.

Test-run data was unavailable for the current build (the supplied test-runs endpoint returned 404); this is a build signing failure, not a named test assertion.

## Root Cause Analysis
**Observed:** `SignToolTask` invokes `.packages/sn/1.0.0/sn.exe` during signing on Darwin and AzureLinux legs, and process startup fails with OS error 8. This recurs across six supplied builds.

**Assessment:** The immediate cause is that the Unix signing environment is given an executable it cannot execute, blocking affected release build legs. The evidence does not establish whether the trigger is RID/package selection, signing configuration, or an incompatible Arcade SDK asset.

**Confidence:** High for the immediate process-format incompatibility; Medium for the broader configuration assessment because no binlog or package metadata is supplied.

**Alternatives / Unknowns:** It is unknown whether `sn.exe` is a Windows-only asset, whether a platform-specific launcher should be selected, or whether the defect belongs in Arcade rather than SDK configuration. No target-branch comparison or Build Analysis result was supplied.

## Suggested Investigation
1. Inspect the resolved RID and file format of `.packages/sn/1.0.0/sn.exe` in failing Darwin/AzureLinux builds; compare with a successful Unix signing build or expected platform asset.
2. Review `Microsoft.DotNet.Arcade.Sdk/.../tools/Sign.proj(59,5)` and SDK signing configuration to determine why this executable is selected on Unix.
3. Compare build progression near the first occurrence and inspect a binlog, if available, to distinguish SDK configuration from Arcade package/tool regression.

The generic failed Build parent and artifact cascades are not separate causes; the actionable mechanism is the specific signing-tool process-start failure.

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