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/README.md

@@ -2,7 +2,7 @@
 
 The canonical source of `~/.pi/agent/extensions/mempalace.ts` — the TypeScript
 extension that wires [MemPalace](https://github.com/MemPalace/mempalace)'s MCP
-server into the [pi coding-agent](https://github.com/mariozechner/pi-coding-agent)
+server into the [pi coding-agent](https://github.com/earendil-works/pi)
 harness. Installs wake-up context injection, per-tool schema passthrough,
 and a `/mempalace-diary` slash-command.
 
@@ -114,7 +114,7 @@ git-crypt unlock <key>
 
 ```bash
 brew install pi-coding-agent       # macOS
-# or see https://github.com/mariozechner/pi-coding-agent for Linux
+# or see https://github.com/earendil-works/pi for Linux
 pi --help                          # creates ~/.pi/agent/
 ```