Define session file migration target

[ComputelessComputer]

Summary:

• Add the durable session Markdown and SQLite migration plan.
• Add the canonical session Markdown parse/render contract with tests.

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Note

Low Risk
Adds a new session Markdown parsing/rendering utility and documentation, but it is not yet wired into runtime session I/O, so impact is limited to new codepaths and test coverage.

Overview
Introduces a canonical, versioned session.md file format for sessions, including frontmatter for stable identity/metadata and # Notes/# Summary/# Transcript sections, plus parseSessionMarkdown/renderSessionMarkdown helpers.

Adds Vitest coverage for round-tripping, loose/hand-authored compatibility (unsectioned bodies treated as notes), nested heading handling, and required metadata errors. Also adds docs/data-migration-plan.md describing the target storage boundaries and a phased migration approach from legacy session folders/TinyBase to Markdown + SQLite.

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