Bug: actionable error (no crash) when scan/build root looks like a whole filesystem

[vibgrate-team]

Context

Walking an accidental root (/, a full OS image tree, or an enormous unpack directory) can hang, OOM, or fail opaquely. Local-first tools should fail closed with an actionable error (what happened + what to pass instead) rather than crashing or silently thrashing the machine.

Related: ignore/exclude docs (#162). This ticket is specifically about hard safety rails and error quality at the root boundary.

What to do

1. Detect clearly unsafe / runaway roots (e.g. filesystem root, obvious OS image layouts, or walk budgets exceeded).
2. Exit non-zero with a deterministic, actionable message: suggest a project subdirectory, --ignore patterns, or raising an explicit override flag if one exists.
3. Add a focused test that asserts the error path (no flake, no wall-clock dependence).

Acceptance

• Unsafe root → clear error + non-zero exit; no crash dump as the primary UX
• Message tells the user what to do next (narrow the path / ignore / override)
• Deterministic tests; DCO; pnpm test green