fix(update): skip unmanaged local files silently, consolidate find loops

[bcdady]

Description

Previously, any file present locally but absent from the remote repo produced a Could not update warning — identical in appearance to a genuine failure. In practice this affects every installation: users commonly have local-only files (custom commands, skills, config) sitting alongside OAC-managed files in the same directory.

Two changes:

1. Silent skip for unmanaged files — a 404 response is no longer treated as a failure. Local-only files are counted and shown in the summary line only.
2. Consolidated find — three sequential find/loop blocks (md, ts, sh) replaced with a single pass.

Also replaces the per-file backup/restore mechanism with a single shared temp file. Downloading to temp before overwriting the destination achieves the same safety guarantee (no partial overwrites) with less code.

Summary line before:

Updated: 3 file(s), failed: 47 file(s)

Summary line after:

Updated: 3 | Skipped (local-only): 47 | Failed: 0

Type of Change

• New feature (agent, command, tool)
• Bug fix
• Documentation
• Refactoring
• CI/CD

Checklist

• Tests pass locally
• Documentation updated (if needed)
• Follows CONTRIBUTING.md

Testing

Ran update.sh --install-dir ~/.config/opencode against a global install directory containing ~130 OAC-managed files and ~16 local-only files. Local-only files were silently skipped and counted; OAC-managed files were updated as expected. No backup files left behind.