fix: remove destructive config-writing from install CLI

Install command was parsing and rewriting the entire opencode.json,
nuking all user config (MCPs, providers, LSP, etc.) every time it ran.

Removed addPluginToConfig, addInstructionsToConfig, and createNewConfig.
Install now only stores the API key and installs the skill — it prints
instructions instead of touching opencode.json.

Constraint: install must never write to opencode.json
Rejected: keep addPluginToConfig with backup | still rewrites full config via JSON.stringify
Confidence: high
Scope-risk: narrow