fix: deduplicate session environment exports

[dinhnguyenminhhoang]

Summary

Prevent repeated SessionStart events from appending duplicate Codex
environment exports to CLAUDE_ENV_FILE.

Fixes #528

Changes

• Skip environment export lines that already exist
• Preserve unrelated environment-file content
• Add regression coverage for repeated SessionStart invocations

Testing

• node --test --test-name-pattern="session start hook does not duplicate" tests/runtime.test.mjs
• Manually invoked the real SessionStart hook three times against a
  temporary environment file
• Verified each managed export appeared exactly once
• Verified unrelated environment entries were preserved

[chatgpt-codex-connector]

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