Add custom step extension API, bump to 1.1.0

[pallaoro]

Summary

Public extension API: registerStepType() lets host applications add domain-specific step types (e.g. signed callouts to internal APIs) alongside built-ins without forking. Custom steps participate in the same do: dispatch, validation, and trace pipeline as native types.

What's new

• StepRegistry — register/get/has/names/clear; module-level defaultRegistry
• registerStepType({ name, allowedKeys, validate?, run }) — collision with built-ins and duplicate registration throw at register time
• CustomStepContext — frozen state, env, prefixed logger, per-flow AbortSignal, nodeName, resolveTemplate()
• Pre-resolves {{ template }} strings in input fields, mirroring do: http
• Validator errors surface with the native { node, field, message } shape — indistinguishable from built-in validation
• PluginConfig.customSteps — pass a private registry for test isolation or multi-runner scenarios

Design notes

• No new schema-library dependency. Validator is (input) => { ok, errors } — plugin authors wrap Zod/Ajv if they want.
• AbortSignal is wired to a per-flow controller now; flow-level cancel can call abort() later without API change.
• No retry helper on ctx — node-level retry: already covers this.
• Module-level registerStepType() writes to a singleton; opt out via cfg.customSteps.

Public API

import { registerStepType } from "@clawnify/clawflow";

registerStepType({
  name: "clawnify_app",
  allowedKeys: ["app_id", "method", "path", "body"],
  validate: (input) => { /* return { ok, errors } */ },
  async run(input, ctx) {
    const res = await fetch(url, { signal: ctx.abortSignal });
    return { status: res.status, ok: res.ok, body: await res.json() };
  },
});

Flow JSON unchanged: { "do": "clawnify_app", "name": "...", "app_id": "..." }.

Test plan

• npm run lint — clean
• npm test — 128 tests pass (11 new + 117 existing)
• Smoke-test against a real consumer plugin before publishing

Release

Merging this PR + cutting a v1.1.0 GitHub release will auto-publish to npm.