fix(readme): restore README doctest coverage broken by PR2 (main red)

[rickylabs]

Fix: main is red — README doctests broken by PR2 (#117)

PR2's README revamp (f68fa916) turned main red on ci / check-test
(3 failed | 874 passed). The doctests that guard the package READMEs
were broken, not the package APIs:

Failing test	Cause
packages/sdk/tests/readme-doctest_test.ts (TS fences)	Usage fence relabeled ```ts → ```typescript; the doctest only extracts ts, so it found zero checkable fences.
packages/sdk/tests/readme-doctest_test.ts (JSON fences)	The README's only ```json fence was dropped; the doctest requires ≥1 parseable JSON fence.
packages/service/tests/_fixtures/readme-examples_test.ts	The createService().withAuthn().serve() / defineService(..., { auth }) example was removed, so the pinned lifecycle substrings were absent.

Fix (README-only — no source or test changes)

• @netscript/sdk — relabel the Usage fence back to ```ts (it type-checks clean against the doctest prelude) and add a ```json import-map fence.
• @netscript/service — restore the proven fluent-builder + auth examples (createService(...).withAuthn({ authenticator }).serve() and the defineService(..., { auth: {...} }) preset opt-in), carrying the lifecycle substrings the fixture asserts.

Validation

deno test --allow-all \
  packages/sdk/tests/readme-doctest_test.ts \
  packages/service/tests/_fixtures/readme-examples_test.ts
# ok | 4 passed | 0 failed

deno fmt --check packages/sdk/README.md packages/service/README.md
# Checked 2 files

Scope is deliberately minimal so main goes green ahead of the JSR
publish; the PR3 root-README work is unaffected.

🤖 Generated with Claude Code