docs: redact secrets in remember skill

[wbugitlab1]

Summary

• update the packaged remember skill to redact credentials and secrets before memory_save
• add a security-sensitive worked example that preserves operational meaning without storing raw token values
• add a plugin-surface regression test for the remember-skill safety contract

Verification

• corepack pnpm test -- test/plugin-surface-contract.test.ts passed: 171 files / 2238 tests
• corepack pnpm exec tsx scripts/skills/check.ts passed: 15 skills
• git diff --check and git diff --cached --check passed
• semgrep scan --config p/default --error --metrics=off . passed: 0 findings
• gitleaks protect --staged --redact passed: no leaks

Refs #919