Make Quantitative Grounding explicit-only

[andydrewie]

What changed

• Makes the installed Codex skill explicit-only through $quantitative-grounding.
• Adds policy.allow_implicit_invocation: false to both metadata mirrors.
• Preserves SYSTEM_PROMPT.txt as a separate operator-controlled persistent deployment mode.
• Updates documentation, evaluations, examples, validation, manifests, and checksums.
• Publishes the incompatible activation-contract change as version 2.0.0.

Why

The repository and runtime policy previously described default/implicit activation, which conflicted with the portfolio’s intentional-invocation convention.

Validation

• Repository validator passed.
• Generated legacy overlay check passed.
• Official root and packaged-skill validation passed.
• git diff --check passed.