Surgical plugin-array config editor

[diegohb]

What to build

A config reader/writer that understands every config pattern OpenCode allows — opencode.json and opencode.jsonc at the scope base and the repo root, plus global config.json (read-only) — and that can ensure a package's plugin array entry with surgical semantics:

• Text-splice insertion, leaving every other byte of the file untouched
• Zero writes when a semantically matching entry already exists (name-aware, @version/@latest-tolerant)
• When no config exists anywhere, create a repo-root opencode.jsonc by default (scope-base equivalent for global scope)
• Warn and abort without modification when a candidate config file is unparseable

This is the capability later tickets port into the CLI and the generated-package templates. Grounded in ADR-0008 (content-based deployment plans) and the Zero-write no-op invariant in CONTEXT.md.

Acceptance criteria

• Ensuring an entry in an existing config with comments/formatting changes only the plugin array; the rest of the file is byte-identical
• Re-running against a config that already contains the entry performs no write at all
• Entries match semantically: bare name, name@version, name@latest
• With no config anywhere, a minimal valid default config file is created with the entry
• Unparseable config files produce a warning and abort with no modification
• Strict .json and lenient .jsonc (comments, trailing commas) both parse; $schema URLs containing // and escaped quotes survive

Blocked by

None (can start immediately).