feat: add dev fetch command

[kiro-agent]

Summary

• add a dev fetch command with raw Git argument forwarding
• refuse fetches while the repository is locked
• document fetch and updated lock behavior

Validation

• npm run build
• dev fetch --dry-run --no-write-fetch-head
• confirmed locked repositories fail before fetching
• confirmed dev checkout fetch --help still shows checkout help

Notes

• npm test -- --runInBand could not run because jest is not provided by the package.