[Feature] Copy markdown by cell

[mwittie]

Describe your changes

Introduces a Cell abstraction that sits between raw markdown Blocks and final output, establishing a 4-step pipeline: parse → classify → execute → compose.

• StaticCell passes content through unchanged; BashCell is stubbed ready for future execution
• ProcessFile orchestrates the full pipeline end-to-end
• Restored fmt, fmt-check, and vet Makefile targets and wired them into pre-pr
• Restored testify v1.11.1 and removed unused sergi/go-diff dependency

Notes for reviewers

Execution is intentionally a no-op in this PR (copy-through). BashCell holds the output field and rendering logic ready for when execution is wired up.

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.