refactor: finish companion rename migration

Complete the remaining pi-to-companion rename across companion-os, web, vm-orchestrator, docker, and archived fixtures.

Verification:
- semantic rg sweeps for Pi/piConfig/getPi/.pi runtime references
- npm run check in apps/companion-os (fails in this worktree: biome not found)

Co-authored-by: Codex <noreply@openai.com>

packages/coding-agent/test/system-prompt.test.ts

@@ -103,7 +103,7 @@ describe("buildSystemPrompt", () => {
       const prompt = buildSystemPrompt({
         contextFiles: [
           {
-            path: "/home/node/.pi/workspace/USER.md",
+            path: "/home/node/.companion/workspace/USER.md",
             content: "# User\n\nLikes coffee.",
           },
         ],