feat: extract @libscope/registry — git-backed pack registry

[RobertLD]

Summary

Part of #488. Extract src/registry/ into a standalone @libscope/registry package. The registry depends almost entirely on Node.js stdlib and is one of the cleanest extractions in the split.

Problem / Motivation

The registry system (git-backed pack sync, publish, resolve, checksum) is a self-contained utility with minimal dependencies on the rest of libscope. It's unrelated to semantic search or document indexing from a consumer's perspective, and shouldn't be bundled with the core search engine.

Proposed Solution

Move src/registry/ into packages/registry/ with its own package.json. The registry depends on @libscope/core only for pack import/export — that interface should be kept minimal.

Acceptance Criteria

• @libscope/registry builds and tests pass independently
• The package depends only on @libscope/core (for pack import/export) and Node.js stdlib (fs, path, crypto, zlib)
• Existing CLI registry commands (libscope registry add, libscope pack install, etc.) work correctly
• Path validation logic (reject .., /, \, null bytes) is preserved and tested

Out of Scope

• Changes to the registry format or protocol
• Adding new registry features

Technical Notes

• src/registry/ contains: config.ts, git.ts, sync.ts, publish.ts, search.ts, checksum.ts, resolve.ts
• Key stdlib deps: fs, path, crypto, zlib — no external npm deps beyond @libscope/core
• Registry names validation: /^[a-zA-Z0-9_-]+$/, 2–64 chars
• Git URLs: https://, ssh://, or git@host:path (no embedded credentials)