refactor(release): share PR context resolution

[jasonkuhrt]

Summary

• introduce a shared explorer GitHub context object and pull-request context resolver
• reuse that context across explorer, PR preview, PR title, and PR-number resolution flows
• add focused helper coverage and document the refactor rationale

Testing

• bun run --cwd packages/release check:types
• bun run --cwd packages/release test packages/release/src/api/explorer/explore-helpers.test.ts packages/release/src/api/explorer/explore.test.ts packages/release/src/cli/pr-preview.test.ts
• bun run release:verify
• git diff --check

Closes #156

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This change is