feat(opencode): cut plugin system and remote behavior feeds; add SessionContext assembly and Runner

- delete plugin machinery (loader, install, meta, hooks) and all trigger sites
- fold first-party provider auth into static registry (provider/hooks.ts)
- remove remote instruction URL fetch, skills remote puller, TUI plugin host
- add session/context.ts: single provenance-tagged context assembly point
- add session/runner.ts: admit/context/stream/tools loop with compaction policy
- relocate audit artifacts to audit/ (FORK-AUDIT, instruction docs as evidence)

packages/opencode/test/skill/skill.test.ts

@@ -2,7 +2,6 @@ import { describe, expect } from "bun:test"
 import { LayerNode } from "@opencode-ai/core/effect/layer-node"
 import { Effect, Layer } from "effect"
 import { Skill } from "../../src/skill"
-import { Discovery } from "../../src/skill/discovery"
 import { RuntimeFlags } from "../../src/effect/runtime-flags"
 import { EventV2Bridge } from "../../src/event-v2-bridge"
 import { Config } from "../../src/config/config"