--lint severity control: --lint-level, inline suppression, per-file allow in eigs.json

[InauguralPhysicist]

Why. --lint exits 1 on any warning including W001 unused-variable (verified live), with no threshold or suppression mechanism. Consumer repos — the forcing functions — cannot wire --lint --json into CI without warnings-as-errors and no escape hatch.

What.

• --lint-level error|warning choosing the exit-1 threshold.
• Inline suppression comment (e.g. # lint: allow W001).
• Per-file allow list in eigs.json.
• Document in docs/DIAGNOSTICS.md; suite coverage in tests/test_lint.sh.

Effort: days. Clean scope — good contributor on-ramp. Source: 2026-07-04 survey (tooling dimension).

[InauguralPhysicist]

Core landed in #428: --lint-level error|warning (exit-code policy in src/lint.c) and inline # lint: allow W0xx suppression are both live and in --help. The remaining mechanism — per-file allow-list in eigs.json — is split out as #455 (kept good-first-issue; it's the contributor on-ramp this issue intended). Closing.

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