Remove docs/history.md

It had grown past 1,700 lines, too large to be read or kept up. What it held --
what was wrong before a change and what it cost to find -- goes in commit
message bodies now, beside the change. CLAUDE.md says so; the README and the
changelog no longer point at it. It remains in git history.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

CLAUDE.md

@@ -154,9 +154,9 @@ addressed to the person using it.
 Every change gets one line under `## [Unreleased]` in [CHANGELOG.md](CHANGELOG.md), in its
 [Keep a Changelog](https://keepachangelog.com/en/1.1.0/) group: Added, Changed, Deprecated,
 Removed, Fixed or Security. Say it the way someone using ipx would notice it. When there is more to
-say, such as what was wrong before or what it cost to find out, write it up at the top of
-[docs/history.md](docs/history.md), dated. That record has been more useful than the git log more
-than once.
+say, such as what was wrong before or what it cost to find out, it goes in the commit message's
+body, where `git log` and `git blame` find it beside the change. (There was a long-form
+`docs/history.md` until 0.7.0; it grew too large to be useful and was removed. It is in git.)
 
 Cutting a release: rename `[Unreleased]` to `## [X.Y.Z] - YYYY-MM-DD` and open a new empty
 `[Unreleased]` above it, bump `version` in `Cargo.toml`, tag the commit `vX.Y.Z`, and update the