feat(cli): show computed checksum during weave publish

[breferrari]

Context

When a pack author runs weave publish, the CLI collects files and creates a registry PR. It does not compute or display the SHA-256 checksum of the pack content. The registry's generate.py independently computes the checksum after merge. If there is a canonicalization bug, it would only surface when someone tries to install — not during publishing.

Proposal

During weave publish, compute and display the pack's checksum:

Publishing webdev v1.2.0...
  Local checksum: sha256:abc123... (registry will independently verify)
  Created PR: https://github.com/PackWeave/registry/pull/5

This gives pack authors a reference value for debugging checksum mismatches.

Acceptance criteria

• weave publish displays the computed checksum before creating the PR
• Checksum is computed using the same checksum::compute() function used by verification