Fix env guard for Windows state paths

[trevor050]

Summary

• Normalize state and base directory paths with the appropriate POSIX or Windows path rules before validating them.
• Preserve valid generated AgentBridge environment variables when native Windows paths use backslashes, including mixed separators and path casing.
• Add regression tests for the default fix guard behavior and Windows path comparisons.

Related to #231. On native Windows, valid AGENTBRIDGE_STATE_DIR values were rejected because the guard only recognized /pairs/<id> and ${baseDir}/ separators. In fix mode that erased the registered pair identity, causing Claude Code to treat the live pair as manual mode.

Validation

• bun test src/unit-test/env-guard.test.ts — passed (8 tests).
• bun run typecheck — passed.
• bun run check — blocked by an unrelated Windows integration failure in broker-graceful-shutdown.test.ts (expected SIGTERM exit code 0, received null).
• bun run test:unit — unrelated existing Windows failures in installer shell stubs and missing Unix tail executable.

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