fix(install): align P2 test fixtures with kernel cleanup and managed-writer contracts

[777genius]

Summary

Fixes all 6 P2 findings from PR 177 review on d5f13af. These are test/fixture defects only; production admission, service.go, writer protocol floor, and product-identity-as-destination are unchanged.

• Windows skill cleanup: last-consumer --remove now asserts the owned skill is absent and consumers are empty (Unix shared-retention half unchanged).
• Prepare permissions: keep exact POSIX 0600/0640 checks on Unix; on Windows assert writable regular file 0666 (identityMode).
• Managed-writer E2E fake: internal-install-runtime --refresh publishes both BAT launcher aliases with production WindowsLauncherScript bytes; refresh still fails if launchers are missing.
• Config E2E installer: replace the copy-only script with production bin/install.sh plus isolated seams (staged/local assets, no network, no desktop integrations) so setup-codex commits a protocol-compliant writer.
• MCP deadline clock: arm now.Store(100) only after synctest.Wait() so initialize/initialized and the deadline watcher cannot consume the capture budget.
• Concurrent snapshot writer: bounded retry on Windows ERROR_SHARING_VIOLATION / Access is denied for the fixture WriteFile+Rename only.

Test plan

• go test ./internal/config/ -count=1 -run TestPrepare
• go test ./internal/installruntime/ -count=1 -run TestSetupSnapshotPreimageIsBoundToParsedBytes
• go test ./internal/agentnotify/mcp/ -count=1 -run TestSDKOriginalDeadlineExpiredBeforeHandler
• go test ./cmd/claude-notifications/ -count=1 -run TestEmbeddedSkillLifecycleProjection
• test_windows_native_hooks_configured_existing_binary from bin/install_e2e_test.sh
• scripts/config_e2e_test.py 16/16 with isolated helper binary
• CI on the PR against feat/agent-notify-e2e (Windows 1.25/1.26, Ubuntu, macOS E2E)

  

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[777genius]

Landed on feat/agent-notify-e2e for PR 177; closing the nested PR.