decree-docs: inject mode for existing READMEs

[zeevdr]

Description

An --inject mode writes generated docs between <!-- BEGIN_DECREE_DOCS --> and <!-- END_DECREE_DOCS --> markers in an existing file, leaving surrounding hand-written content untouched (terraform-docs model). This suits keeping a schema section fresh inside a project README via CI or pre-commit.

Acceptance criteria

• Injection replaces only the marked region and fails with a clear error when markers are missing or unbalanced.
• Running injection twice on the same input is a no-op (idempotent).
• Test coverage: unit tests for marker handling (missing, unbalanced, multiple) and an idempotency test.