Add preface outline placeholder for third edition framing

Co-authored-by: Scott Chacon <schacon@gmail.com>

Author: cursoragent

book/preface.asc

@@ -0,0 +1,35 @@
+[preface]
+== Preface to the Third Edition
+
+////
+AUTHOR TODO: outline notes only — the actual preface prose must be written by a
+human author before publication (see AGENTS.md). Replace everything below with
+real prose.
+////
+
+[NOTE]
+====
+*Draft placeholder — outline notes only; prose not yet written.*
+====
+
+Points this preface needs to make:
+
+* *Timing*: edition written _before_ the Git 3.0 release
+** based on the announced/committed 3.0 breaking changes (see the
+   https://git-scm.com/docs/BreakingChanges[BreakingChanges document])
+** no firm 3.0 release date at time of writing; details could still shift
+* *Stance*: the book treats the *new 3.0 defaults as the defaults* throughout
+** `main` as the default branch name
+** SHA-256 as the default object hash for new repositories
+** `reftable` as the default ref storage backend
+** `safe.bareRepository` = `explicit`
+* *Dual paths*: some sections deliberately cover *both* the old and new behavior,
+  because readers will live with both for years
+** SHA-1 vs SHA-256 (existing repos stay SHA-1; interop story)
+** `files`/packed-refs vs `reftable` ref storage
+** `master`-named branches in older/existing repositories
+* *Readers on Git 2.x*: what to expect if their installed Git predates 3.0
+  (which defaults differ, how to opt in early — e.g. `init.defaultBranch`,
+  `--object-format`)
+* *Caveat*: commands still marked experimental at time of writing
+  (e.g. `git history`) may change by/after 3.0

progit.asc

@@ -10,6 +10,8 @@ Scott Chacon; Ben Straub
 
 include::book/license.asc[]
 
+include::book/preface.asc[]
+
 include::book/introduction.asc[]
 
 include::book/ch01-getting-started.asc[]