feat: interactive disk reclaim via scoped sweep and leftover-target purge

[cs50victor]

• Gate the sweep suggestion on cargo-sweep being on PATH; when missing, offer the install instead, so frd never proposes a step that can only fail
• Replace the fixed project-root sweep with a scope picked at accept time: the project dir or any parent up to the home dir, recursing for the wider scopes
• Non-interactive and --yes runs take the narrowest scope (project dir)
• Report reclaimed space by sizing the chosen directory before and after the sweep
• Doctor now lists the sweep as recurring maintenance via the new SweepSpec

Validation: cargo test (5 pass, +2 for the parent-dir boundary), fmt check, and dry-run plus doctor smoke tests with the binary present and absent