feat(skills): add Pythonic code guidance

[phernandez]

Why

The recent local/cloud runtime consolidation made Basic Memory more capable, but it also exposed how easily portable Python can accumulate protocol towers, helper chains, misleading ownership names, and duplicated source-of-truth assumptions.

We want a shared review and implementation lens for code that is clear to both human and AI contributors: Pythonic without being clever, explicit about domain authority, and conservative about abstraction.

What Changed

• add the /pythonic-code skill with write, refactor, and review modes
• prefer functions and typed values before hierarchies, while retaining concrete classes and narrow protocols for real state and runtime boundaries
• add docs/DOMAIN_MODEL.md to define projects, notes, entities, observations, relations, NoteContent, identity, ownership, and local versus DB-first authority
• expand the engineering style guide with an explicit abstraction budget and private-helper-sprawl guidance
• point AGENTS.md at the domain model and summarize the new house style

Implementation Details

This is guidance rather than a mechanical style mandate. Correctness, compatibility, and existing repository conventions remain higher priority than applying an idiom.

The domain model keeps Markdown as the canonical product representation while distinguishing durable local file authority from the accepted NoteContent state used during DB-first materialization.

Testing

• uv run python /Users/phernandez/.codex/skills/.system/skill-creator/scripts/quick_validate.py .agents/skills/pythonic-code — passed
• just package-check — passed
• just agent-harness-check — passed
• git diff --cached --check — passed before commit

Risks / Follow-ups

No runtime behavior changes.

Follow-up cleanup is tracked in:

• refactor(core): clarify shared runtime package ownership and dependency direction #1096 — clarify shared runtime package ownership and dependency direction
• refactor(core): make accepted note snapshot persistence indivisible #1097 — make accepted note snapshot persistence indivisible
• refactor(core): simplify the accepted-note preparation and protocol surface #1098 — simplify the accepted-note preparation and protocol surface

These should land incrementally rather than turning this PR into a broad refactor.