feat: add HCL/Terraform structural extractor

[mauricewittek]

Adds tree-sitter-based extraction for Terraform .tf and .tfvars files, producing block-level graph nodes and cross-file module dependency edges.

What it does

• Extracts all 7 HCL block types (resource, data, module, variable, output, locals, provider) as graph nodes with stable namespaced IDs
• Creates containment edges (file → block) and module_source edges (module → referenced source)
• Resolves cross-file module-input edges (module call argument → child variable definition)
• Classifies .tf/.tfvars as code in detect.py and routes them through the HCL extractor in the dispatch loop
• Handles parse errors gracefully with structured diagnostics
• Enforces resource limits (5MB file size, 200k AST nodes)

I tried to implement this in small commits to make it easier to review commit-by-commit.