fix(security): avoid retaining dynamic regexes

[alxxjohn]

Motivation

• Prevent unbounded process-wide growth of compiled regular expressions originating from attacker-controlled TypeScript aliases and namespaces in long-lived SDK/MCP processes by removing a package-global cache.

Description

• Remove the package-level dynamicPatternCache and change compileDynamicPattern to directly call regexp.MustCompile so dynamic expressions are not retained across scans and can be garbage-collected.

Testing

• Ran go test ./internal/codeguard/checks/security ./internal/codeguard/runner/... and go test ./..., and all test packages executed successfully (no failures).

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Codex Task