AGENTS: note pi changelog source is npm-tarball CHANGELOG.md

Companion to opencode-devbox's 'Upstream sources' section. Pi's npm
package ships a rich CHANGELOG.md with New Features / Added / Changed
/ Fixed sections — but the npm registry metadata ('npm view') doesn't
include the changelog body. Surface the 'npm pack + tar' recipe in
the release-day checklist so future-pi (and human-pi) doesn't try to
derive notes from npm view alone.

Doc-only, no CI implications.

AGENTS.md

@@ -23,6 +23,19 @@ Container image that adds pi coding-agent on top of the opencode-devbox base ima
 3. Add fresh `## Unreleased` section
 4. Commit, tag `vX.Y.Z`, push tag → CI fires automatically
 
+When drafting CHANGELOG entries, pull pi's release notes from the
+`CHANGELOG.md` shipped inside the npm tarball:
+
+```bash
+cd /tmp && npm pack @earendil-works/pi-coding-agent@<version>
+tar -xzf earendil-works-pi-coding-agent-<version>.tgz package/CHANGELOG.md
+head -40 package/CHANGELOG.md
+```
+
+Pi's CHANGELOG has rich New Features / Added / Changed / Fixed sections
+per version. Don't try to derive notes from the npm registry metadata
+(`npm view`) — it doesn't include the changelog body.
+
 ## Key facts
 
 - **Base image**: `joakimp/opencode-devbox:base-latest` — rebuilt whenever opencode-devbox cuts a new base