Rename @mariozechner/pi-coding-agent to @earendil-works/pi-coding-agent

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).

The old @mariozechner/pi-coding-agent npm package is deprecated with
the explicit message 'please use @earendil-works/pi-coding-agent
instead going forward', and the version stream has moved on (old
top-out 0.73.1; new currently 0.74.0). Anyone npm-installing the old
name today gets a deprecation warning + a stale binary, so this is
a non-optional migration before the next tagged release.

Sweep:
- Dockerfile (production single-Dockerfile path) and Dockerfile.variant
  (split-base path on main): npm install -g target updated.
- README, AGENTS, HUB_TEMPLATE: github.com/mariozechner/pi-coding-agent
  URL refs (which now 404) -> github.com/earendil-works/pi.
- DOCKER_HUB.md regenerated (5529 bytes, ~78% headroom).
- CHANGELOG Unreleased: rename entry added with migration context.

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

Historical CHANGELOG entries: only github URL refs updated (the
package name was never spelled out in those entries; we're correcting
dead hyperlinks, not rewriting feature descriptions).

Dockerfile.variant

@@ -48,9 +48,9 @@ ARG PI_TOOLKIT_REF=main
 ARG PI_EXTENSIONS_REF=main
 RUN if [ "${INSTALL_PI}" = "true" ]; then \
       if [ "${PI_VERSION}" = "latest" ]; then \
-        NPM_CONFIG_PREFIX=/usr npm install -g @mariozechner/pi-coding-agent ; \
+        NPM_CONFIG_PREFIX=/usr npm install -g @earendil-works/pi-coding-agent ; \
       else \
-        NPM_CONFIG_PREFIX=/usr npm install -g @mariozechner/pi-coding-agent@${PI_VERSION} ; \
+        NPM_CONFIG_PREFIX=/usr npm install -g @earendil-works/pi-coding-agent@${PI_VERSION} ; \
       fi && \
       pi --version && \
       git clone --depth 1 --branch "${PI_TOOLKIT_REF}" \