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.

install.sh

@@ -244,7 +244,7 @@ install_pi_extension() {
     if [[ ! -d "$PI_EXT_DEST_DIR" ]]; then
         note "pi not detected at $PI_EXT_DEST_DIR — skipping pi extension"
         printf '       (install pi first if you want the pi↔mempalace bridge:\n'
-        printf '        https://github.com/mariozechner/pi-coding-agent)\n'
+        printf '        https://github.com/earendil-works/pi)\n'
         return 0
     fi