feat: add memory validation layer

[wbugitlab1]

Summary

• add a dependency-free memory validation layer with shadow, block, and disabled modes
• validate explicit memory, lesson, and slot writes before persistence or mutation, with standalone MCP fallback coverage
• document AGENTMEMORY_MEMORY_VALIDATION and refresh the generated config skill reference

Verification

• corepack pnpm exec vitest run test/memory-validation.test.ts test/remember-project-scope.test.ts test/lessons.test.ts test/slots.test.ts test/mcp-standalone.test.ts
• corepack pnpm exec vitest run test/cross-project-isolation.test.ts test/worktree-project-scope.test.ts test/plugin-surface-contract.test.ts
• corepack pnpm run lint
• corepack pnpm test
• corepack pnpm run build
• semgrep scan --config p/default --error --metrics=off .
• corepack pnpm run skills:check
• git diff --check
• gitleaks protect --staged --redact

Closes #340