gate --help coverage across vrg-* tools + convention doc (#2036)
Adds tests/vergil_tooling/test_help_coverage.py: enumerates [project.scripts], runs each covered tool with --help in a subprocess, and asserts exit 0 + non-empty output. vrg-hook-guard is EXEMPT (a hook, not a human CLI); tools that do not yet answer --help are tracked in KNOWN_GAPS, and test_gap_set_matches_reality keeps that list in sync with the source so fixing a tool forces its removal and a new help-less tool fails the gate. Documents the convention under standards/development and wires the nav. Ref vergil-project/vergil-tooling#2030.