Align agent discovery test with exact-case README.md handling

[Copilot]

macOS / Electron failed because a unit test expected lowercase readme.md to be excluded from .github/agents, while runtime classification excludes only exact-case README.md. This updates the test to match the implemented case-sensitive rule.

• Failure addressed

  • SessionCustomizationDiscovery test incorrectly treated readme.md as documentation-only.
  • Actual behavior: only README.md is excluded; other .md files in agent folders remain agent candidates unless filtered by more-specific types.

• Changes

  • Renamed the test to make the rule explicit: exact-case README.md exclusion.
  • Seeded lowercase readme.md as a regular agent file input.
  • Updated expected discovery output to include lowercase readme.md and continue excluding exact-case README.md.

• Resulting contract

  • Platform-side discovery expectations now match workbench prompt classification for agent-folder markdown files.

await seed('/workspace/.github/agents/README.md', 'docs');      // excluded
const wsLowerReadmeAgent = await seed('/workspace/.github/agents/readme.md', 'docs lower'); // included