Add a plugin tool

A plugin tool is declared in the plugin's plugin.json, handled by its isolated backend process and exposed to every engine as <plugin-id>__<tool-name>. Contract reference: docs/plugins.md (manifest table, API versions, consent model) and src/plugin-sdk/index.d.ts.

Files

1. Manifest - src/plugins/<id>/plugin.json, tools[]: {name, description, parameters:{type:"object", properties, required}}.
   • name: lowercase id, unique within the plugin.
   • Parameters are scalar (string|number|boolean) under API 1 and 2; object only with apiVersion: 3. Anything else is rejected by validateManifest (src/substrate/plugins/manifest.ts).
   • Add confirmation (1-300 chars, API 2+) when the tool spends money, publishes or otherwise needs the user's consent before each run.
   • The description is model-facing: say when to use it, what it costs and what not to do.
2. Backend - the tool(name, args, ctx) handler returned by activate() in the plugin's backend (src/plugins/example/backend.mjs is the minimal pattern; src/plugins/genex/backend.ts, src/plugins/blender/backend.ts are real ones).
   • Treat args as untrusted: check types and ranges again, reject unknown operations.
   • Reach host services only through ctx.host(...); a backend cannot read credentials, open URLs or touch files outside the bound project and its own storage.
   • Return plain JSON. Throw an Error with an actionable message on refusal.
3. Skill text - if the agent needs guidance, update the plugin's skills[] entry so it names the tool by its full <id>__<tool> name.
4. Host changes (only when changing the tool system itself): validation in src/substrate/plugins/manifest.ts, dispatch/consent in src/substrate/plugins/registry.ts, the call, its consent wait and its durable record in src/main/core/plugin-tools.ts (PluginToolService), and engine exposure in src/main/core/delegation.ts (search pluginTools: the tool list handed to a session and the dispatch of its calls).
5. Docs - docs/plugins.md for contract changes; docs/product/assets-plugins.md for anything a user sees.

Tests (red first)

• Registry behaviour against a copied package: tests/conformance/plugins.test.ts (fixture() copies src/plugins/example into a temp root) - declared vs undeclared fields, wrong types, disabled plugin fails closed, cancelled call not retried.
• Manifest rules: validateManifest cases in the same file.
• Consent: tests/conformance/plugin-consent.test.ts when confirmation is involved.
• Genex/Blender tools: tests/conformance/genex-plugin-cli.test.ts, tests/conformance/blender.test.ts.
• Package as a user would see it: npm run plugin:doctor -- src/plugins/<id>.

npm test -- tests/conformance/plugins.test.ts tests/conformance/plugin-devkit.test.ts
npm run plugin:doctor -- src/plugins/example

Never let a tool argument choose a program, URL or path outside the project; those come from reviewed plugin code.