Extract error handling, parsing logic, and variable substitution into dedicated
modules. This reduces duplication between tui.json and opencode.json parsing
and makes the config system easier to extend for future config formats.
Fixes potential plugin resolution issues when switching between projects by wrapping
plugin loading in Instance.provide(). This ensures each plugin resolves dependencies
relative to its correct project directory instead of inheriting context from whatever
instance happened to be active.
Also reorganizes config loading code into focused modules (command.ts, managed.ts,
plugin.ts) to make the codebase easier to maintain and test.
