fix: sync pkg/package.json version with pi-coding-agent to prevent false update banner

[alphinus]

Problem

gsd-pi sets PI_PACKAGE_DIR=pkg/ so that pi's config.js reads piConfig from pkg/package.json (for branding). However, config.js also reads version from that same file and uses it for the update check — comparing it against the npm registry.

pkg/package.json had a hardcoded "version": "0.1.0" that was never updated when new versions were published. This means:

• VERSION in pi's internals is always 0.1.0
• The update check compares 0.1.0 against the latest npm version (e.g. 0.57.1)
• Result: "Update Available" banner always appears, even when the user is already on the latest release

Note: PI_SKIP_VERSION_CHECK=1 was added in a recent commit and suppresses this in newer releases, but the root cause (stale version) remains and could cause issues anywhere VERSION is used.

Fix

1. Updated pkg/package.json version to match the current pi-coding-agent dependency (0.57.1)
2. Added scripts/sync-pkg-version.cjs — reads the installed @mariozechner/pi-coding-agent version and writes it into pkg/package.json
3. Integrated into prepublishOnly so the version is automatically synced on every npm publish

Testing

# Script correctly detects and updates stale version
$ node scripts/sync-pkg-version.cjs
[sync-pkg-version] Updating pkg/package.json version: 0.1.0 → 0.57.1

# Script is idempotent when version already matches
$ node scripts/sync-pkg-version.cjs
[sync-pkg-version] pkg/package.json version already matches: 0.57.1

Affected versions

All published versions of gsd-pi (0.1.x through 0.2.8) show the false update banner when PI_SKIP_VERSION_CHECK is not set.