chore(release): group the changelog and polish the README

[KristofferRisa]

Prep for the v0.4.2 release. Two parts: release notes that are worth reading, and a README audit.

Release notes

changelog gained grouping, and the release body gained a header and an install footer. Verified by dry run (goreleaser release --skip=publish,announce), which caught three things I would otherwise have shipped blind:

• chore(release): leaked into the notes. The existing filter was ^chore:, which does not match a scoped prefix. All five prefix filters now accept an optional scope.
• Contributor commits were misfiled. The conventional-commit regexps missed a plain Fix segmentation fault ... subject, so Fix segmentation fault on invalid home-id in live command #17 landed under "Other changes" instead of "Bug fixes". The group regexps now match a bare Fix / Feat subject case-insensitively, and a catch-all group guarantees nothing is silently dropped.
• 40-char SHAs were noise. Now abbrev: 7 — GitHub autolinks 7+ char hashes in a release body, so each entry gets a clickable commit link.

Rendered output for v0.4.2:

## Changelog
### 🐛 Bug fixes
* cfdf37e Fix segmentation fault on invalid home-id in live command
### 🔧 Other changes
* c04440a Refactor websocket live errors and test duplication

I also set use: git explicitly rather than github. The compare API adds a network dependency that can fail a release for no real gain, and the local git log is reproducible in a dry run.

README

• Title said "Tibber CLI" — the project, module, and binary are all powerctl.
• --home-id was undocumented on both live and consumption, despite being the flag at the centre of panic: segmentation fault in powerctl live with invalid home ID #13.
• Troubleshooting gained the invalid or non-existing home ID error that Fix segmentation fault on invalid home-id in live command #17 introduced, so the message is greppable.
• Version example claimed 1.0.0, which has never shipped.

Verification

• goreleaser check passes
• make check passes (fmt, vet, mod verify, race tests)
• Dry-run build produces a working binary: powerctl version → 0.4.2

⚡ Built on gilfoyle with Claude Code