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)

docs/dev/development.md

@@ -21,7 +21,7 @@ We use `just` as our primary command runner. Recipes are organized into modular
 - `justfiles/test.just` (`[test]` group): `test`, `lint`, `analyze` (alias `ana`)
 - `justfiles/gen.just` (`[gen]` group): `gen-docs`
 - `justfiles/pkg.just` (`[pkg]` group): `pkg`
-- `justfiles/debug.just` (`[debug]` group): `debug`, `debug-update`, `debug-notify`, `debug-install`
+- `justfiles/debug.just` (`[debug]` group): `debug`, `debug-update`, `debug-changelog`, `debug-notify`, `debug-install`, `debug-autoupdate`
 
 Run `just -l` to quickly view all available recipes organized by group.