chore(registry): remove dead TarballVersionCollisionError, record CL-7587 end state

[TheGreatAxios]

Summary

• Removes the dead TarballVersionCollisionError from @corbits/tool-registry-publish (never thrown: publishes skip an existing name@version; never caught: grep-zero).
• Records the CL-7587 pack + explicit-publish end state in the package README, including what blocks full deletion.
• Full package deletion is blocked: @corbits/seeding, @workbench/onboarding, and scripts/checks/tool-package-freshness import this package's library entry (their lanes own those moves). No boot-time publish, no daemon/freshness loop, no registry table, pins resolve on immutable name@version.

Verification

• bun run check: exit 0 (typecheck, lint, test, check:structural).
• packages/tool-registry-publish bun test standalone: 78 pass, 0 fail.
• grep TarballVersionCollisionError across repo: zero matches.
• Pre-push hook suite on push: 284 pass, 0 fail.

Fixes CL-7587

[linear-code]

CL-7587