fix: safely handle trailing commas in OpenCode config

[thaikolja]

Summary

Allows the install and uninstall scripts to read an existing opencode.json containing trailing commas.

Root cause

The scripts used strict JSON parsers. The initial fallback used a regular expression that also matched comma-like text inside quoted values, which could silently change user configuration.

Changes

• Parse strict JSON first and only normalize after a parse failure.
• Remove trailing commas with string- and escape-aware scanners in Node.js and PowerShell.
• Preserve values such as "literal,}" and "literal,]" exactly.
• Add functional install/uninstall regression tests for POSIX and Windows PowerShell.
• Run installer tests on Ubuntu and Windows in GitHub Actions.
• Update official GitHub actions to their Node 24-based major versions and restrict workflow permissions to read-only contents.

Verification

• python scripts/validate-package.py
• Bash syntax checks for install and uninstall scripts
• POSIX functional installer test
• PowerShell 7 functional installer test
• Windows PowerShell 5.1 functional installer test
• GitHub Actions run 31328829070: all three jobs passed

Original trailing-comma fix contributed by @thaikolja; safety hardening and regression coverage added by the maintainer.