Add Resend migration verifier CLI

[jaeyunha]

Closes #643.

Summary

• Adds private in-tree @opensend/migrate-from-resend package with migrate-from-resend CLI.
• Scans a target repo for common Resend SDK imports/calls, Resend env vars, and raw REST endpoint usage.
• Maps findings through a committed compatibility matrix with honest full, partial, unsupported, and unknown statuses.
• Generates a shareable Markdown report with discovered usage, status counts, env/base URL diffs, caveats, evidence references, and rerun commands.
• Adds sandbox request planning only via --sandbox-plan --base-url ... --api-key ...; no network calls, no sends, and no contact/domain/customer mutations.
• Adds public “Migrate from Resend” docs page and regenerates /docs/llms.txt.

v1 boundaries / caveats

• Static scan is advisory and regex-based; dynamic wrappers/generated endpoints may need manual review.
• No codemods or automatic rewrites.
• No historical data migration from Resend.
• No npm publishing in this PR; package is private: true with no publish hook.
• No live sends/mutations. Sandbox mode renders planned requests only and redacts auth.
• CLI fails closed unless a target directory is supplied; --sandbox-plan requires explicit --base-url or OPENSEND_BASE_URL.

Report / dry-run evidence

Manual fixture command:

bun run --cwd packages/migrate-from-resend migrate-from-resend tests/fixtures/resend-app \
  --sandbox-plan \
  --base-url http://localhost:3015 \
  --api-key os_test \
  --output /tmp/opensend-migrate-report.md

Observed output:

Scanned 4 files, found 22 Resend compatibility findings (12 partial/unsupported/unknown).
Summary in report: Full 10, Partial 7, Unsupported 1, Unknown 4.
Sandbox request plan entries are `planned-not-sent` and use `Authorization: Bearer <redacted>`.

Guardrail evidence:

bun run --cwd packages/migrate-from-resend migrate-from-resend
→ target-dir is required; pass the application repository you want to scan

Validation

• bun run --cwd packages/migrate-from-resend test ✅ (4 files / 9 tests)
• bun run --cwd packages/migrate-from-resend typecheck ✅
• bun run docs:generate ✅ (217 markdown docs indexed)
• bun test tests/docs-content.test.ts ✅ (14 tests)
• make check ✅ (TypeCheck + Biome over 829 files)
• make test ✅ (215 files passed, 3 skipped; 1754 tests passed, 10 skipped)
• git diff --check HEAD~1..HEAD ✅
• Independent code-review lane: APPROVE, 0 issues.
• Independent architecture recheck: CLEAR after fixing same-line unknown SDK-call false negative.