feat: add opt-in Hub share build

[luhe19001]

Summary

This PR adds a fixture-backed Share to Hub flow for reviewing the future opt-in sharing experience.

• select sessions by repository and time window
• review the exact canonical payload before approval
• bind consent to the source revision, payload hash and size, and destination workspace
• invalidate stale reviews and ignore late preview responses
• preserve unchanged retries while requiring renewed approval when the source or destination changes
• exclude accepted items from partial retries and expose the canonical Hub route after success

Try it locally

Append ?team-share=1 to the local coSlash URL.

• &share-state=signed_out
• &share-state=pairing_required
• &share-state=credential_dormant
• &share-state=credential_revoked
• &share-result=partial

Scope

This is a clearly labeled NO UPLOAD build harness. It does not add real authentication, credential storage, ingestion, persistence, or navigation. Production acceptance remains gated on the end-to-end cloud integration.

Verification

• 23 frontend tests
• production frontend build
• frontend lint and format checks
• go test ./...
• go vet ./...
• git diff --check
• no Go *_test.go files changed

Browser QA remains pending.