CON-51: a branch does not change the same thing twice

[molgerz]

Closes CON-51.

What this adds

One bullet in the Conventions section of AGENTS.md, directly after the
gates bullet: a branch does not change the same thing twice. Its commits
have to read as if the work had been done once, in order — no file touched by
two commits unless the second is a consequence of later work rather than a
correction of the first, and nothing a branch adds is renamed or removed again
by that same branch.

It carries the check as a command, so nobody has to invent it:

git log --format='%h' main..HEAD | while read c; do git show --format= --name-only $c; done | sort | uniq -c | sort -rn

Anything above 1 is a file to justify or to fold together. The repair named
in the rule is to rebuild the branch from its final tree rather than patch
history — the churn then disappears by construction — with
git diff --quiet HEAD <backup> as the proof that the reviewed tree survived.

Why it sits next to the gates

Because it leans on them. A split is only honest if every commit passes
typecheck, lint, test and build on its own, and that same requirement
is what caps the number of commits: a boundary that would split one file
across two commits is not a boundary at all.

Where it came from

CON-11 (#94). Four commits; four files touched by three of them, sixteen by
two. Two files were created and deleted again inside the branch —
src/ui/hide-page.ts became src/ui/archive-page.ts,
publish-page.tombstone.test.ts became publish-page.archive.test.ts —
neither having ever existed on main, so every line of both was noise for a
reviewer. Rebuilt from the final tree: two commits, each file exactly once,
identical tree. Nothing of the content was lost; the whole difference was
reviewability.

Scope

Documentation only. CLAUDE.md is a symlink to AGENTS.md, so there is no
second copy to keep in step. No code, no tooling: a hook could count the
touches, but the judgement — is this a correction or a consequence? — is not
mechanical, so it stays a written rule for now.

Verification

Nothing to test in the browser. typecheck, lint, test and build all
green (vitest run src: 577 files, 5670 tests).