feat: jspod install <app> — one-command Solid app installs - #40

Fixes #36.

What

`jspod install ` replaces the multi-step git-push recipe new users had to copy-paste. Defaults to `github.com/solid-apps/` as the source; one command per app or batch by listing several.

```bash
jspod install chrome # one
jspod install vellum win98 pdf # several
jspod install # curated set: chrome vellum win98 pdf hub
jspod install --pod http://other.pod chrome
jspod install --help
```

How it hides the sharp edges

The manual recipe had three friction points that all bite new users on first use:

1. Shallow clones get rejected. `git push` from a `--depth=1` clone fails with `shallow update not allowed`. The install does a full clone.
2. HEAD/branch mismatch. `updateInstead` (which extracts the working tree) only fires when the push targets the branch HEAD points at on the server. JSS 0.0.197+ auto-inits with HEAD=main, but older versions honor the operator's `init.defaultBranch` (often `gh-pages` for GitHub-Pages-heavy users). The install pushes both `HEAD:main` and `HEAD:gh-pages` — whichever matches HEAD extracts; the other just creates a ref. Idempotent on re-run.
3. Already-bundled apps (pilot). jspod ships pilot at `/public/apps/pilot/` skip-if-exists. Auto-init refuses the push because the path is non-empty. The install translates the cryptic `repository not found` into a friendly `⊘ pilot: skipped` rather than scaring the user.

Verified end-to-end

• `jspod install pilot` → `⊘ skipped (path already in use)`, exit 0
• `jspod install alarm` on the running pod (JSS 0.0.196, init.defaultBranch=gh-pages) → `✓ alarm → http://localhost:5444/public/apps/alarm/\`, `index.html` reachable (200) with real content
• `jspod install --help` shows the subcommand-specific help
• `jspod --help` mentions `install` so users discover it

Diff

• `index.js`: +155 LoC (subcommand dispatch, `runInstall`, `printInstallHelp`, main `--help` updated)
• `README.md`: +14 LoC ("Install Solid apps" section)

Follow-ups (separate issues)

• Support git sources in jspod install (jspod install <repo>) #25 — generalize to arbitrary GitHub repos (`jspod install solid-apps/penny#v2`, `jspod install someone/cool-thing`)
• Generalize "installing apps" into a CLI command: jspod install &lt;source&gt; #24 — generalize further (non-git sources, registry, etc.)