fix(blueprint-metadata): resolve bare github.com URIs to raw metadata JSON

[tangletools]

Summary

• Several blueprints just registered on Base Sepolia (e.g. ai-agent-sandbox, llm-inference, modal-inference, image-gen-inference) carry an on-chain metadataUri that points at the repo's GitHub HTML page (https://github.com/<owner>/<repo>). The dapp fetch()-es that URL expecting JSON, receives HTML, parseBlueprintMetadataJsonText bails out, and every affected blueprint renders as unavailable.
• Teach resolveBlueprintMetadataFetchUrl to translate a bare github.com/<owner>/<repo> URL (optionally with a trailing slash or .git suffix) into raw.githubusercontent.com/<owner>/<repo>/main/metadata/blueprint-metadata.json. ipfs:// handling is untouched, URLs with sub-paths (/tree/main/...) are passed through unchanged, and any other HTTPS URL still flows through rewriteLocalhostUrlForBrowser.
• Repos that default to master instead of main will 404 for now — that's accepted as a follow-up (fixable on-chain via updateBlueprint, or we can add a main → master fallback later). The four repos with on-chain registrations today all default to main.
• The resolver moves into its own metadataFetchUrl.ts so unit tests don't transitively pull in the authoring module's import.meta.env reference; authoring.ts re-exports the function so the public API is identical.

Test plan

• yarn nx test tangle-shared-ui --testPathPattern=authoring — 6 new cases (ipfs, bare GitHub, trailing slash, .git, sub-path passthrough, unrelated HTTPS passthrough), all pass.
• yarn nx test tangle-shared-ui — full suite green (32 tests).
• yarn nx test tangle-cloud — full suite green (109 tests).
• yarn nx lint tangle-shared-ui — clean.
• yarn nx build tangle-cloud — clean.
• Manual smoke on Base Sepolia: hit the four affected blueprint detail pages and confirm metadata loads.

[drewstone]

Approved.