add agent guidance, comment convention, and skill contract guards

[DonIsmaelito]

Adds AGENTS.md so coding agents know the repo's rules, puts a short plain comment above every function in the existing helpers, and adds two tests: one that keeps the comment convention, one that keeps SKILL.md's numbered hard rules in order and checks every path it references exists. Also a pytest path setting so plain pytest works, and a one-line fix in the timeline PNG export.

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Summary by cubic

Adds repo guidance and contract guards so concurrent agents can work safely without silently breaking the public editing rules, and standardizes Python definition comments plus fixes timeline PNG extraction compatibility. The branch is merged with main and preserves existing caption behavior.

• AGENTS.md documents product principles, architecture boundaries, workflow, and branch discipline.
• Every Python file now requires a module docstring and plain comment text above each function or class, enforced by tests/test_comment_style.py.
• tests/test_skill_contract.py verifies numbered hard rules stay in order and every referenced path or helper script exists.
• Adds a pytest pythonpath setting so plain pytest works from the repo root.
• Timeline PNG extraction now forces the yuvj420p pixel format.

^{Written for commit b600b1c. Summary will update on new commits.}

[cubic-dev-ai]

All reported issues were addressed across 12 files

<sub>Reply with feedback, questions, or to request a fix.

Fix all with cubic | Re-trigger cubic</sub>

[cubic-dev-ai]

All reported issues were addressed across 3 files (changes from recent commits).

<sub>Reply with feedback, questions, or to request a fix.

Fix all with cubic | Re-trigger cubic</sub>