Add @Plugin reference example

[brsbl]

What was wrong

Core had the primitives needed to mention plugins, but no in-repository reference consumer demonstrated the complete intended behavior. The reviewed example also ranked description-only installed-plugin matches alongside identity matches, allowing a description hit to outrank a plugin name or ID substring, and it did not preserve plugin IDs as host-ranked identity aliases.

What changed

Adds examples/plugins/at-plugin, a backend-only reference plugin named @Plugin with an AtIcon asset and two providers on the normal @ trigger:

• Installed reads live plugin inventory, keeps running plugins with an agent-facing skill, tool, or CLI, and ranks exact name/ID, prefix name/ID, substring name/ID, then description-only matches.
• Community searches bb's reviewed bb-community catalog and returns only compatible, uninstalled entries, with the same identity-first tiers ahead of catalog-only fallback matches.

Both providers emit bounded plugin-ID aliases through experimental_searchAliases; they do not supply numeric ranks. Selection is revalidated at send time. Installed mentions attach advisory context for capabilities already available to the agent, while Community mentions explicitly remain unavailable until the user installs them. Mentioning never grants installation consent or invokes a target plugin.

The example keeps version 0.1.0, engines.bb >=0.0, and engines.bbPluginSdk >=0.4.8. It changes no shared SDK/version/engine floor and does not ship or submit a marketplace plugin.

How you verified

• Replayed conflict-free onto parent head f1f39af8d30848e875ca50286f666b0ff35e2c68; the layer's stable patch ID is unchanged. Every required GitHub CI job passed at exact head 6f636f5ceb00c2e1155eefd6b680021951d4c32f.
• Final example run: 4 files, 72/72 tests across identity/description ranking, alias propagation, catalog eligibility, context bounds, provider isolation, timeouts, send-time revalidation, and no target invocation.
• Post-rebase Turbo typecheck passed all 4 tasks for bb-plugin-at-plugin; direct ESLint and Prettier passed.
• Exact final-bottom and final-child desktop dev apps used the same fictional project, route/state, and copied 1280×900 native window bounds. The child installed only the exact checked-in example. Five repeated query cycles, four viewport captures, and transition widths 389/390/391, 767/768/769, 1023/1024/1025, and 1919/1920/1921 all passed with no horizontal overflow. Full immutable evidence manifest.

Before — the parent has no @Plugin provider

After — the exact example adds the installed-plugin result first

Fixes: no issue; this is the reference-intent layer for the @Plugin core stack.

BB-Thread-ID: thr_94xb8a4nk2

AGENT GENERATED: by GPT-5.6