feat: update changelog (#131)

closes #130 

- CHANGELOG.md file for people who can view changelog from github
---
`iris changelog` command:
`iris changelog [version]` (e.g. iris changelog v0.5.1)
`iris changelog -n 3` (see changelog of 3 latest releaseas)

=> `iris changelog` looks just like CHANGELOG.md, grouped headers and
all. It only stops looking that way if we personally overwrite that
release's description with something else
=> With 1-hour local cache (invalidated on updater-channel change),
`--refresh` flag to bypass, graceful fallback to stale cache on GitHub
rate-limiting

---
- New auto update config
```toml
# 0 = off (default, notify only), 1 = auto-install, 2 = always confirm first
auto-update = 1
```

- New just debug-changelog and just debug-autoupdate recipes (fully
offline, mocked HTTP servers, no real network/config touched)

.github/workflows/nightly.yml

@@ -33,8 +33,10 @@ jobs:
           IFS='.' read -r major minor patch <<< "$VERSION_NUM"
           patch=$((patch + 1))
           NEXT_BASE="v${major}.${minor}.${patch}"
-          SHORT=$(git rev-parse --short HEAD)
+          SHORT=$(git rev-parse --short=6 HEAD)
           echo "version=${NEXT_BASE}-nightly.${SHORT}" >> $GITHUB_OUTPUT
+          echo "short_sha=${SHORT}" >> $GITHUB_OUTPUT
+          echo "subject=$(git log -1 --format=%s)" >> $GITHUB_OUTPUT
 
       - name: Build Binaries
         run: |
@@ -71,12 +73,18 @@ jobs:
         with:
           tag_name: ${{ steps.version.outputs.version }}
           name: ${{ steps.version.outputs.version }}
+          # shaped like GoReleaser's changelog groups so `iris changelog` renders it
           body: |
             # 🌙 **Nightly build** - `${{ steps.version.outputs.version }}`
-            Nightly release from commit: ${{ github.sha }}
 
-            # Update: 
-            `iris update`
+            ### Nightly
+
+            * ${{ steps.version.outputs.short_sha }}  ${{ steps.version.outputs.subject }}
+
+            ## Update
+            ```bash
+            iris update
+            ```
 
             > install manually, this build will **not** trigger an update notification
           files: dist/*.tar.gz