[FEATURE]: Configurable plan file directory with opt-in in-project storage #51304

Feature hasn't been suggested before.

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Describe the enhancement you want to request

In Plan mode, generated plan markdown files always go to the global ~/.opencode location, even when the session is running inside a git-backed project root.

As a developer I want a project-level setting to opt into in-project storage, e.g. <project-root>/.opencode/plans/ or a custom relative path like docs/plans/, so that all my project related planning is centrally traceable.

Expected: config option with current global path as default; when set, Plan mode writes/reads plans from the project directory so plans can be versioned and shared per repo.