feat: add dependency file triggers for terraform matching

[vedant-rox]

Summary

This PR is the new first reviewable slice of #791.

It adds an opt-in dependency_file_triggers feature for Terraform/OpenTofu projects that narrows changed-file matching by following local module imports recursively and adding the imported module *.tf* files to each project's trigger patterns.

Scope

This PR intentionally stays within docs/ and libs/digger_config/.

Included here:

• YAML/config support for dependency_file_triggers
• trigger-pattern discovery for local Terraform/OpenTofu modules
• generated-project support for Terraform/OpenTofu generation paths
• validation for unsupported Terragrunt/Pulumi usage
• docs and regression tests

Explicitly not included here:

• inferred project dependency edges
• hard dependency propagation changes
• GitHub/GitLab event semantics
• Terragrunt trigger discovery or hardening

Safety

• the feature is opt-in via dependency_file_triggers: true
• existing include_patterns / exclude_patterns behavior remains in place
• exclude_patterns still take precedence over derived trigger paths
• generated Terraform/OpenTofu projects can opt in
• Terragrunt and Pulumi reject dependency_file_triggers explicitly in this slice

Validation

• cd libs && env GOTOOLCHAIN=local GOTELEMETRY=off GOCACHE=/tmp/digger-go-cache GOMODCACHE=/tmp/digger-go-modcache /opt/homebrew/opt/go@1.25/bin/go test ./digger_config/...

Docs

• updated the config reference for dependency_file_triggers
• updated the include/exclude patterns guide with the narrower dependency-file-based triggering option

Follow-up

The inferred dependency-edge / hard-mode propagation behavior is being kept as a separate follow-up PR because that is where the recent review feedback and CI-semantic risk has been concentrated.

AI disclosure

This PR was prepared with assistance from Codex (OpenAI).