Native Kotlin/Wasm SDK for Golem agents

[JohnSColeman]

Summary

Adds a native Kotlin SDK under sdks/kotlin/ for writing Golem agents in idiomatic
Kotlin. Agents — and the SDK itself — compile directly to a WebAssembly Component via
Kotlin/Wasm (WasmGC): no JavaScript, no QuickJS, no engine embedding. The SDK links the
Golem host interfaces (golem:agent, golem:api/*, golem:durability, wasi:*, …) through
the raw canonical ABI, producing a small, interpreter-free component.

What's included

• sdk/ — core SDK module: canonical-ABI runtime/guest, host bindings, hand-rolled codecs
  (zero new runtime deps).
• ksp/ — KSP annotation processor: reads @Agent/@Endpoint/… and generates the
  @WasmExport guest entry points + agent metadata.
• gradle-plugin/ + wit-native/ — Gradle plugin driving the Kotlin/Wasm compile and the
  wasm-tools embed → new → validate pipeline, plus the bundled WIT.
• example/ — a durable counter agent.
• README.md + docs/ — programming-model docs and a capability matrix.
• scripts/ — host-level e2e + contract-test harnesses.
• cli/golem-cli — Kotlin language support: golem new --template kotlin and
  kotlin/snapshotting, first-class Kotlin agent-id/value rendering + parsing
  (SourceLanguage::Kotlin), and Kotlin build-tool requirements (JDK 17).
• golem-skills/…/golem-custom-snapshot-kotlin — snapshotting skill.
• Host WasmGC config — enable WasmGC in the wasmtime configs of golem-worker-executor,
  golem-component-compilation-service, and agent-type extraction (required to run native
  Kotlin/Wasm components).

Capabilities

Cross-referenced against the Scala reference SDK — 15 complete, 3 partial:

• Complete: agent model, Kotlin⇄WIT type mapping, host API, oplog, retry + DSL,
  transactions, context, RPC (@RemoteAgent typed clients), RDBMS, quota, WASI, guards &
  checkpoint, secrets, snapshotting (typed Snapshotted<S> mixin, KSP-derived codec),
  utility types.
• Partial: durability (lazy-initialized-pollable deferred), tools (streamed stdin
  deferred), middleware (request-time HTTP enforcement is host-side).
• Not started: bridge SDK generation for Kotlin (a golem-cli code-gen gap — does not
  affect in-Golem RPC); request-time HTTP routing/enforcement (host-side).

Testing

• Unit: 98 Kotlin SDK tests (sdk 60 / ksp 30 / gradle-plugin 8) + golem-cli suite
  green (incl. Kotlin agent-id round-trip tests).
• Integration (real toolchain, local Golem server): scaffold→build→deploy→invoke e2e;
  15/15 ABI-boundary contract probes; and a manual snapshot-update round-trip proving typed
  agent state survives a component revision bump and a server restart.

Notes

• Snapshotting is typed-only (Snapshotted<S>) — the codec is derived by KSP from the
  state type; there is no raw-bytes hook path. On a manual update the agent is reconstructed
  from its own agent-id inside load-snapshot.
• WasmGC must be enabled host-side for these components to run (included in this PR).

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