Rename to @earendil-works/pi-coding-agent + earendil-works/pi URL

Pi moved to its new home at earendil-works on 2026-05-07
(https://pi.dev/news/2026/5/7/pi-has-a-new-home).

Sweep:
- extensions/pi/mempalace.ts: 'import type { ExtensionAPI } from
  "@mariozechner/pi-coding-agent"' -> @earendil-works/pi-coding-agent.
- README and extensions/pi/README: github.com/mariozechner/pi-coding-agent
  URL refs -> github.com/earendil-works/pi.
- install.sh: same URL substitution in the user-facing pointer line.

Brew install references (`brew install pi-coding-agent`) left as-is:
formula still works at 0.73.1, tap update tracked upstream at
earendil-works/pi#2755.

extensions/pi/mempalace.ts

@@ -25,7 +25,7 @@
  */
 
 import { type ChildProcessWithoutNullStreams, spawn } from "node:child_process";
-import type { ExtensionAPI } from "@mariozechner/pi-coding-agent";
+import type { ExtensionAPI } from "@earendil-works/pi-coding-agent";
 import { Type } from "typebox";
 
 // Minimal MCP stdio JSON-RPC client. MCP uses newline-delimited JSON.