ci(audience-sdk): stop running Unity tests on docs-only PRs (SDK-268)

[ImmutableJeffrey]

Summary

• Doc-only changes (e.g. adding a README.md under src/Packages/Audience/) currently trigger the full Audience CI surface — test-audience-sample-app.yml runs all 12 self-hosted Unity PlayMode cells, and test-audience-sdk.yml runs the .NET unit-test job.
• Both workflows scope their paths: filter to the entire package and example directories (src/Packages/Audience/**, examples/audience/**), with no carve-out for documentation or repo metadata.
• This PR replaces those blanket matches with explicit allow-lists of source-affecting roots, so doc-only PRs no longer schedule the matrix.

test-audience-sdk.yml now triggers on:

• src/Packages/Audience/Runtime/**
• src/Packages/Audience/Tests/**
• src/Packages/Audience/package.json
• src/Packages/Audience/Directory.Build.props
• src/Packages/Audience/link.xml
• .github/workflows/test-audience-sdk.yml

test-audience-sample-app.yml triggers on the same paths, plus:

• examples/audience/Assets/**
• examples/audience/Packages/**
• examples/audience/ProjectSettings/**
• .github/workflows/test-audience-sample-app.yml

paths and paths-ignore are mutually exclusive in GitHub Actions, and paths-ignore only suppresses a workflow when every changed file matches the ignore list — so a mixed change (markdown + package.json) would still trigger. An explicit allow-list is the only filter shape that handles mixed PRs correctly.

package.json, Directory.Build.props, and link.xml stay in the trigger because version, dependency, build-config, and IL2CPP-stripping changes can each produce regressions the matrix exists to catch.

Linear: SDK-268

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