[Feature]: Promote plugin to Stable — freeze the hook-bus / DI / directive APIs #145

Pre-flight checklist

• I searched existing issues and did not find a duplicate request.
• I explained the problem or use case, not just the desired API.

Problem or use case
plugin is Beta and is the extensibility contract the whole ecosystem would build on — yet its surface is one minor old: the hook bus, DI container, async install, namespaced directives, and unuse/uninstall all arrived in 1.14.0. Third-party plugin authors cannot target a moving extension API; the contract must be frozen before an ecosystem can form.

Proposed solution
Freeze the plugin API (hook bus, DI container, install/uninstall lifecycle, namespaced directive registration) for at least one minor cycle and publish a plugin-author guide documenting the lifecycle, hook timing, DI resolution rules, and directive-namespacing conventions. Add coverage for install/uninstall symmetry (no leaked hooks/directives after uninstall). Exit criteria: frozen surface, author guide, tested lifecycle symmetry.

Possible API or UX shape

import { definePlugin } from "@bquery/bquery/plugin";

export default definePlugin({
  name: "my-plugin",
  async install(ctx) {
    ctx.provide("token", service);
    ctx.directive("my:thing", handler);
    ctx.hook("view:updated", fn);
  },
  uninstall(ctx) { /* must cleanly remove all of the above */ }
});

Alternatives considered
Promoting before freezing — would strand early plugin authors on a changing contract. Expanding the hook surface further first — fine while Beta, but it must stop for a cycle to earn Stable.

Relevant area
plugin

Additional context
A stable plugin API is a prerequisite for any community ecosystem, which is itself one of bQuery's adoption gaps versus the established frameworks. Lifecycle symmetry (clean uninstall) is the highest-risk area to test.