chore: automate README docs generation and formatting

Manual `terraform-docs markdown .` was fragile — no injection markers,
no idempotency check, and no consistent formatting pipeline.

Add terraform-docs inject mode with BEGIN/END_TF_DOCS markers in the
README so `terraform-docs .` regenerates only the managed section.
Wrap the injected block with prettier-ignore directives to prevent
prettier from reformatting tables (which would break terraform-docs
--output-check).

Restructure Makefile with three targets:
- `docs`: regenerate terraform-docs + prettier format README
- `fmt`: docs + terraform fmt -recursive
- `check`: verify all docs/formatting are current (CI-friendly)

Add node (LTS) and prettier to .mise.toml for markdown formatting.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Makefile

@@ -1,4 +1,13 @@
-.PHONY: docs
-.SILENT: docs
+.PHONY: docs fmt check
+
 docs:
-	terraform-docs markdown .
+	terraform-docs .
+	prettier --write README.md
+
+fmt: docs
+	terraform fmt -recursive
+
+check:
+	terraform-docs --output-check .
+	prettier --check README.md
+	terraform fmt -check -recursive