Add git retry logic, sources registry, and configurable knot

Git operations:
- Add retry logic with exponential backoff for HTTP 5xx errors
- Retry on RPC failures, curl errors, connection issues
- 3 retries with 2s/4s/8s delays before failing
- Apply to clone, fetch, pull, push, and subtree operations

Sources registry (sources.toml):
- New module for tracking package URL overrides
- default_url_base for deriving URLs from subtree names
- Per-package entries with url, upstream, branch, reason

Verse config:
- Add knot field for git push server (e.g., "git.recoil.org")
- Default to "git.recoil.org" if not present in config
- Auto-write knot to config file if missing

URL handling:
- Remove hardcoded anil.recoil.org references
- Derive git push server from handle or use configured knot
- Generic SSH URL normalization in forks.ml

Sync resilience:
- Skip failed clones in merge/subtree phases instead of crashing
- Track successfully fetched repos separately from failures

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>