fix: require fork before implementation

[eshulman2]

Summary

• create or reuse the durable fork during workspace setup, before any implementation handoff can push
• checkpoint fork owner/repository and install the fork remote before creating the implementation branch
• fail workspace setup explicitly when the fork cannot be created instead of advancing into a guaranteed persistence failure
• sync the detected default branch rather than assuming main

Testing

• workspace setup suite: 9 passed
• ruff check: passed

Supersedes #201 by adding the missing failure-path guarantee.