feat(allowlist): add HCL/Terraform (.hcl, .tfvars) support #522

Sub-issue of #470 — HCL / Terraform language group.

Scope

• internal/config/allowlist/supported_file_types.json — add .hcl, .tfvars
• internal/config/allowlist/allowed_ext_test.go — test cases for both extensions
• internal/config/rules/rule_docs/terraform.md — dedicated review rules covering hardcoded secrets/credentials, overly permissive network/IAM rules, missing lifecycle protection on stateful resources, and committed state files
• internal/config/rules/system_rules.json — map .tf, .hcl, .tfvars to the new terraform.md doc (.tf currently falls through to the generic default rule set with no dedicated Terraform guidance, despite already being allowlisted — bringing it under the same doc as the two new extensions gives all three consistent, language-specific coverage rather than leaving .tf behind)
• internal/config/rules/system_rules_test.go — rule-resolution test cases

No test-exclude patterns needed per the tracking issue's own table.

I'll follow up with a PR shortly.