Add Claude Code newline-preservation parity tests

[anusbutt]

Context

The shared integration planner promises byte-for-byte preservation of guidance outside its marked block. Codex has focused CRLF and no-final-newline coverage; Claude Code should have explicit parity coverage.

Scope

Extend tests/integration/agent-integrations.test.ts with Claude-specific LF, CRLF, and no-final-newline install and refresh cases.

This is a testing and agent-integration task. Do not duplicate planner logic.

Acceptance criteria

• Tests cover creating and refreshing the Claude block for LF, CRLF, and no-final-newline guidance files.
• Content outside GraphKeeper markers remains byte-for-byte unchanged.
• Repeated integration stays idempotent.
• Malformed markers still fail before any write with GK004.