math-tools: validate-propositions default `_meta.json` path resolves into the plugin cache — R7/R11/R12 silently SKIP without --meta

[kiki830621]

Problem

validate-propositions.py resolves its default _meta.json location from its own file path:

REPO_ROOT = Path(__file__).resolve().parent.parent

When the script runs from the installed plugin cache (~/.claude/plugins/cache/psychquant-claude-plugins/math-tools/<ver>/scripts/), the default _meta.json points inside the plugin cache, not at the target repo. R7 / R11 / R12 then silently SKIP (meta-dependent checks) while the summary still reads like a full pass.

Expected

Default --meta derives from the --jsonl argument's directory (sibling _meta.json), or the script fails loudly when meta is missing — never a silent SKIP that renders as success.

Actual

A run without --meta prints a plausible "checks passed" summary with three checks silently skipped. Observed in practice: a downstream repo reported "ALL VALIDATION CHECKS PASSED" for days before noticing R7/R11/R12 had never run.

Impact

Silent verification gap; the failure mode is invisible precisely because it looks like success. (Same family as the "success signals not derived from checked state" pattern.)