Add preface outline placeholder for third-edition framing

[schacon]

• I provide my work under the project license.
• I grant such license of my work as is required for the purposes of future print editions to Ben Straub and Scott Chacon.

Changes

• Add book/preface.asc — a "Preface to the Third Edition" containing outline notes only (clearly marked as a draft placeholder), to be replaced with author-written prose. The outline captures the framing points: the edition is written before Git 3.0 is released, but treats the new 3.0 defaults (main, SHA-256, reftable, safe.bareRepository=explicit) as the defaults, and some sections deliberately cover both paths (e.g. SHA-1 vs SHA-256).
• Wire the preface into progit.asc between the license and the introduction.

Context

Per AGENTS.md, no reader-facing prose was written — the file is fragment bullets plus an AUTHOR TODO block comment (which does not render), and a visible NOTE admonition marking it as a draft placeholder.

Verified with a full bundle exec rake book:build (all formats build; only the pre-existing external-link check failures, which book:build ignores) and in the local reader:

  

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