feat(install): support arbitrary GitHub repos, refs, and renames - #42

Fixes #25. Builds on the install subcommand from #40.

What

Three input shapes accepted:

Two optional suffixes apply to any shape:

• `#` — pin a branch or tag (uses `git clone --branch `)
• `=` — rename the pod path, useful when the repo name isn't a clean slot

Verified end-to-end

```
$ jspod install litecut/litecut.github.io=litecut
✓ litecut/litecut.github.io=litecut → http://localhost:5444/public/apps/litecut/
```

• `jspod install chrome` — backward compatible (solid-apps default)
• `jspod install litecut/litecut.github.io=litecut` — clones from a non-solid-apps org and renames the pod path; HTTP 200 verified
• `jspod install --help` reflects the new spec
• README updated with new examples

Diff

• `parseAppSpec()` — ~40 LoC, handles all three shapes + `#ref` + `=name` + validation
• Loop refactored: `app` → `input`, uses `spec.{source, name, ref}`
• `git clone --branch ` when pinned
• Banner no longer hardcodes "solid-apps"

Follow-up

`#24` (non-git sources) stays open as the long-term umbrella.