feat(cli): add jss install <name> subcommand (Phase 1 of #464) - #479

Fixes #478. Phase 1 of the phased install plan (#464 comment).

What

`jss install ` clones `github.com/solid-apps/` and pushes it into the running pod at `/public/apps//`. Server operators using `jss start` directly no longer need a wrapper to install Solid apps.

```bash
jss install chrome
jss install vellum pdf hub # multi-arg
jss install chrome --pod http://other.host:5444 # remote pod
```

Implementation

Direct port of jspod's proven `runInstall` (~120 LoC). New file: `src/cli/install.js` exporting `runInstall(names, options)`. Subcommand wired into `bin/jss.js` via commander `.command('install <names...>')`.

The hard infrastructure (git auto-init `-b main`, regular-repo extraction via `updateInstead`, ACL-gated push) already lives in JSS — this is just the orchestration layer.

Behavior

• Source resolution: `` → `https://github.com/solid-apps/` (Phase 1 hardcoded)
• Auth: `POST /idp/credentials` for bearer token. If IDP isn't running (404), proceed without — supports `--public` mode pods
• Clone: Full clone, no `--depth` (shallow pushes are rejected by JSS git-receive)
• Push: Dual `HEAD:main` and `HEAD:gh-pages` so it works regardless of operator's `init.defaultBranch`. Whichever matches server-side HEAD triggers `updateInstead` and extracts the working tree
• Skip-on-existing: Auto-init's "won't init non-empty path" 404 translated to a friendly `⊘ : skipped` message
• Idempotency: Existing repo accepts the push normally; re-runs are safe
• Output: Per-app `✓` / `⊘` / `✗` lines + summary
• Exit code: 1 if any install failed; 0 if all succeeded or all were skipped

Flags

Verified end-to-end against a fresh `jss start`

• `jss install chrome` → `/public/apps/chrome/index.html` returns 200
• `jss install vellum pdf` → both installed, multi-arg works
• `jss install chrome` (re-run) → idempotent, no error
• `jss install ../etc/passwd` → validation error, exit 1
• `jss install nonexistent-name-12345` → clone fails, exit 1
• `jss install chrome --password wrong` → 401 auth fails, exit 1
• `jss install --help` → subcommand help renders cleanly

Depends on

• #477 — port default fallback (`--pod` default `http://localhost:4443\` aligns with the `config.js` default; the leftover `3000` fallbacks in `src/server.js` should be fixed separately)

Not in scope (later phases of #464)

• Phase 2: `/` shorthand, full URLs, `#` pinning, `=`
• Phase 3: `--did did:nostr:` resolution
• Phase 4: `--nostr-privkey ` NIP-98 signed install
• Phase 5: curated default set (no-arg `jss install`)
• Phase 6: `--bundle ` (apt-style meta-packages)

Diff

• `bin/jss.js` +24 (new subcommand block)
• `src/cli/install.js` +205 (new file, the runInstall implementation)