core: consolidate shared infrastructure into core package

Moves effect logging, observability, runtime utilities, flags, installation
version info, and process utilities from opencode to core package. This
enables better code sharing across packages and establishes core as the
single source of truth for foundational utilities.

All internal imports updated to use @opencode-ai/core paths for consistency.

packages/opencode/src/plugin/github-copilot/copilot.ts

@@ -1,6 +1,6 @@
 import type { Hooks, PluginInput } from "@opencode-ai/plugin"
 import type { Model } from "@opencode-ai/sdk/v2"
-import { InstallationVersion } from "@/installation/version"
+import { InstallationVersion } from "@opencode-ai/core/installation/version"
 import { iife } from "@/util/iife"
 import { Log } from "../../util"
 import { setTimeout as sleep } from "node:timers/promises"