005b T1: affected_probes — map a diff to a probe set nf-2ih6 ← Beads

Twinned from https://github.com/cfsh/ninefold/issues/875 by tools/beads/import_github.py's reconcile pass.

Implement T1 from design_docs/005b-affected-probe-selection.md (§3, PR #863).

Goal: a pure function (plus thin CLI) that, given a set of changed file paths, returns the probe names the change can affect.

• Logic in tools/nflib/probe_select.py; CLI tools/pr/affected_probes.py (--paths a b c or two shas via nflib.git), prints affected probe names one per line (C3/C4); --verbose names the matched rule per probe (C5).
• Five rules, first match per path, union across paths — see design doc §3 T1 for the full rule table (probe-file / harness-input / godot_inert / games/<g>/ / fail-closed-all).
• Harness-input tuple guarded by a premise test (SlnPremiseTest pattern in probe_gate.py) that regex-scans imports and fails if one is missing from the tuple.
• Tests: tools/tests/test_probe_select.py, table-driven over all five rules + the premise test. PR body shows the selection output for the last ~40 merges on main (E5).
• One PR: logic, CLI, and tests together — nothing depends on this yet.

Tier-2 tooling: tests required, ./presubmit.py gates, normal review loop.