feat: add --auto-scope flag to scope commands to the current git diff

[amannocci]

🧑‍💻What is the change being made?

Adds a --auto-scope/-A flag to plan, apply, destroy and docs. When set, it computes the files changed relative to HEAD (working tree + staged changes, plus untracked files) via a new Git bind, maps each changed file to its nearest ancestor resource-group directory, and scopes the command to just that set instead of requiring an explicit path argument. --auto-scope and an explicit path are mutually exclusive and raise a UsageError if combined. docs was also refactored to route through the shared resource-dir discovery (resolve_resource_dirs) instead of its own inline directory walk, and its docs-dir cleanup no longer wipes the whole output tree on a scoped run.

❓ Why is the change being made?

CI pipelines and developers working across large conf/resources trees currently have to manually figure out and type the right subpath to limit which resource groups a command touches. This is repetitive and error-prone once a branch spans multiple unrelated directories. Scoping automatically to "whatever changed in this diff" removes that manual step.

✅ How has this been tested?

• New unit tests in tests/it/test_helpers.py covering auto_scope_resource_dirs (staged/unstaged/untracked changes, nested files mapping to their owning group, changes outside the resources dir being ignored, no-changes, not-a-git-repo) and resolve_resource_dirs (mutual exclusion with path).
• New e2e tests in tests/e2e/test_plan_command.py, test_apply_command.py, test_destroy_command.py and test_docs_command.py, using a throwaway git-managed copy of fixture trees to exercise --auto-scope end to end, including the docs rmtree-guard behavior.
• uv run poe lint, uv run basedpyright, and uv run poe test all pass (328 tests).
• Manual smoke testing against a scratch git-managed conf dir with real terraform, confirming correct scoping, the UsageError on combined flags, and a clean error message when run outside a git repository.

📚 How has this been documented?

The flag is self-documenting via its --help text (shared auto_scope_option in commands/helpers.py); this repo has no separate CLI reference doc to update.

🔗 Related Issues

Closes #65