fix: load hook settings from env file

[davidrobertson]

Summary

• Load AgentMemory's env file before every standalone hook reads configuration.
• Preserve process environment precedence over file values.
• Keep generated plugin hooks self-contained and cover env-file-only injection.

Validation

• npm run build
• npm test: 161 files passed, 1 skipped; 1,726 tests passed, 1 skipped.
• A live compiled session-start hook with inherited injection settings removed returned a 4,702-byte AgentMemory context block.
• Isolated QA and code review passed for commit 05cea633441cb3051bedd6a0a1dac2b47a10d633.