fix(agent): detect case-insensitive Windows configuration collisions

[dvd233]

What changed

Normalize prepared configuration target paths for duplicate detection on Windows while preserving the original targetPath for file operations.

Add regression coverage for the Windows path identity helper and for two agent configuration directories that differ only by case but resolve to the same physical directory.

Why

On Windows, realpathSync can preserve the caller's casing for a missing file beneath an existing directory. Two spellings of the same configuration directory therefore produced different targetPath strings and bypassed assertDistinctConfigurationTargets, allowing one setup to overwrite another agent's configuration target.

Validation

• npx tsc --noEmit
• npx eslint src/ test/ (0 errors; one pre-existing warning in test/sdk/speech.test.ts)
• git diff --check
• Windows Node runtime probe: the unpatched path comparison returned NO_ERROR; the patched branch raised the duplicate-target error
• bun test not run because Bun is unavailable in the validation environment

Fixes #257

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