Improve secure code review crypto agility evidence

[DENGXUELIN]

Summary

Closes #1675.

Adds a focused cryptographic agility and migration readiness gate to secure-code-review so reviewers check whether acceptable crypto today can be safely migrated later.

Changes

• Adds SCR-CRYPTO-AGILITY-01 through SCR-CRYPTO-AGILITY-07 findings for missing artifact metadata, scattered crypto constants, missing migration paths, indefinite legacy fallback, non-fail-closed behavior, incomplete migration fixtures, and missing legacy telemetry.
• Extends the report output with a cryptographic agility evidence table.
• Adds vulnerable and benign fixtures covering brittle crypto migration versus versioned crypto migration evidence.

Validation

• git diff --check origin/main...HEAD
• Added-line ASCII check
• Markdown fence balance check for changed files
• Content marker checks for the new evidence gate, finding IDs, and fixtures
• git merge-tree --write-tree origin/main HEAD