all: trim the history essays out of the longest comments

From the audit (#5179). Ten comment blocks narrated how the code got here:
milestone numbers, earlier values, the operator's verdict on an old design.
Each now says what the code does and why, and the history stays in git,
Scribe and docs/sync.md. The protocol-version comment in sync.py points at
docs/sync.md's policy section, which already lists every bump.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

core/src/sync/compat.rs

@@ -187,14 +187,9 @@ pub fn evaluate(info: &ServerInfo) -> Compatibility {
 
 /// Who this client says it is, set once by the host application at startup.
 ///
-/// THE CORE CANNOT KNOW THIS, and the value it used to invent was wrong twice. It
-/// was `inkwell-desktop/{CARGO_PKG_VERSION}`, and this crate is compiled into
-/// the desktop app AND the Android app — so every phone in the field announced
-/// itself as a desktop. The version was worse: `CARGO_PKG_VERSION` here is the
-/// version of the CORE crate, a number no build stamps and no user has ever seen,
-/// while the thing a reader of that header wants is the app's own build (note 3127
-/// §5 — with no version tags, the artifact's self-report is the only answer to
-/// "which build is this?").
+/// The core can't know this: it is compiled into both the desktop and the Android
+/// app, and its own crate version is a number no build stamps. The app's build is
+/// what a reader of the header wants (note 3127 §5).
 ///
 /// So the host names itself. `OnceLock` because identity is fixed for the life of
 /// the process and a second caller should be ignored rather than race the first.