Apply file changes via opencode's patch format (GPT models only).

Wrap in:
  *** Begin Patch
  …file sections…
  *** End Patch

Each section header:
- `*** Add File: path` — body is +-prefixed initial contents.
- `*** Delete File: path` — no body.
- `*** Update File: path` — body is hunks. Optionally `*** Move to: new-path`.
  Hunks: `@@ context` starts. ` ` unchanged, `-` removed, `+` added.
  `*** End of File` closes a hunk to EOF.

Paths are relative to project root.
