Add runnable examples, each smoke-tested against a stub

[garretpremo]

Refs #20. The issue was closed as not needed, but in-repo examples still help the repo: CI now breaks when an SDK change breaks a realistic program, newcomers have something they can run in one command, and the JS SDK has an examples/ directory too.

What's here

Four examples under examples/. Each is its own Gradle project with a short README, a run task, and a smoke test.

Example	Shows
ticket-triage	Keyed questions, models listing, and API errors; a port of the JS SDK's demo.ts
typed-routing	Ask handles and an enum Choice, routed to a queue by an exhaustive switch
async-batch	systemOneAsync fan-out with per-call options, a custom retry policy, and failures per item
spring-boot	The starter behind POST /tickets/route, with TypeSafe failures mapped to 502/504 ProblemDetail

TYPESAFE_API_KEY=your-key ./gradlew :examples:ticket-triage:run

• Testing: each example takes its client through run(TypeSafeClient), so its test points it at examples/support's in-process StubTypeSafeServer. The Spring test sets typesafe.base-url instead. ./gradlew build checks all four with no API key and no secrets in CI.
• README samples: these are output from real runs against the API.
• Publishing: the root build's shared block now configures only typesafe-sdk and typesafe-sdk-spring-boot-starter, so nothing under examples/ is published. A dry run of publishToMavenCentral touches only those two modules.
• Java level: the examples target Java 17, the same as the SDK.

Follow-ups found while writing these (not in this PR)

• Root README, Spring Boot section: typesafe.api-key=${TYPESAFE_API_KEY} doesn't fail fast when the variable is unset. Boot binds the literal ${TYPESAFE_API_KEY} as the key, so the first request gets a 401. ${TYPESAFE_API_KEY:}, as the starter's Javadoc describes, is the form that works.
• Root README, Async section: an exceptionally callback receives a CompletionException wrapping the TypeSafeException, not the exception itself. Only get()/join() hand back the unwrapped cause.
• Possible API additions: ChoiceAnswer.probability() for the chosen label, and a systemOne(state, RequestOptions, Ask...) overload so per-call options don't force the builder form.