Addresses #5958.
Adds `changelogPath` to `PiManifest`. Safe package-relative changelog
resolution, with `CHANGELOG.md` fallback, and carries it through
`PathMetadata`/`SourceInfo`.
Added extension changelog handling like Pi’s own changelog.
If the package exposes `pi.changelogPath` or has `CHANGELOG.md`, Pi
stores the last seen package version in settings. On each fresh
interactive launch, we compare the stored version to the installed
package version. Then parses the changelog entries in between, shows an
Extension Updates section, then marks the current version as seen. The
first time an extension is seen, its version is recorded without showing
a changelog, same as Pi's own on a fresh install. `collapseChangelog`
condenses this to one line per package.
Any update that bumps the extension's version shows this on the next
launch, e.g.:
```bash
pi update --extensions
pi update --all
pi update <extension-source>
pi update --extension <extension-source>
pi update --self --extensions
pi update pi --extensions
pi update self --extensions
```
<img width="798" height="626" alt="Screenshot 2026-08-28 at 16 34 15"
src="https://github.com/user-attachments/assets/c882f01c-94b9-4857-b9b7-243d88c0d437"
/>
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