refactor(plugin): move tool implementation to plugin

[kitlangton]

Summary

• move the canonical V2 Tool implementation into packages/plugin
• keep Core compatibility by re-exporting the plugin-owned Tool from @opencode-ai/core/tool/tool
• narrow ctx.tool.register to Tool.AnyTool and remove the PluginHost cast
• point sdk-next Tool exports at @opencode-ai/plugin

Verification

• bun typecheck in packages/plugin
• bun typecheck in packages/core
• bun typecheck in packages/server
• bun typecheck in packages/sdk-next
• bun run test -- test/session-runner-tool-registry.test.ts test/plugin.test.ts test/tool-shell.test.ts test/tool-subagent.test.ts test/location-layer.test.ts in packages/core
• bun run test -- embedded.test.ts in packages/sdk-next
• pre-push bun turbo typecheck