Fix plugin code quality issues (dead code, cross-platform notifications, E2E test)

[striderZA]

Several plugin code quality issues need attention:

1. Dead code in changelog-generator.ts: updateChangelogFile() is defined but never called. The session.idle handler generates changelogs but only logs them — never writes to disk.

2. showNotification() is Windows-only: Uses PowerShell NotifyIcon for toast notifications. Silently fails on macOS/Linux with no fallback.

3. E2E parity test is a stub: tests/e2e/test-parity.test.ts has describe/it structure but functions return hardcoded paths — never actually spawns OpenCode or Pi.

Tasks

• Either wire up updateChangelogFile() in the session.idle handler or remove the dead code
• Add macOS (osascript) and Linux (notify-send) fallbacks to showNotification()
• Implement actual E2E parity test that spawns both harnesses and compares behavior
• Add timeout to execSync calls in changelog-generator to prevent hangs

Affected Files

• .opencode/plugins/changelog-generator.ts
• .opencode/plugins/ccgs-hooks.ts
• tests/e2e/test-parity.test.ts

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Identified during comprehensive framework review on 2026-07-21