[Feature] Update litdoc file to take a file path

[mwittie]

Describe your changes

• Move the testscript TLD fixtures under cmd/testdata so they live with the CLI package tests.
• Add cmd-level tests for the file command, including negative cases for missing args and missing path.
• Move shared test harness (TestMain, runScript) into testscript_test.go.
• Remove the root testscript harness and its dependency on make build.
• Remove the block subcommand until an internal implementation exists.
• Change litdoc file to accept a path argument instead of reading from stdin, and add a -w/--write flag for in-place rewrite via atomic temp+rename.

Notes for reviewers

• litdoc file now takes a path argument (litdoc file <path>) rather than reading stdin. This is an intentional CLI change to support in-place rewriting with -w.
• The -w flag writes atomically: temp file in the same directory, chmod to preserve mode, then os.Rename.

Checklist before requesting a review

If any of these checks are missing, please provide an explanation.

• I have updated README files, if applicable.
• This PR is small, otherwise explain why.