Rename GitTally to Werkator

`gitTally` is the name of another product in the git space, so the
rename is a precaution; nothing about what the build system does changes.

The name follows one rule: `Werkator` where it is prose, capitalized
where it is a Kotlin type and its file, lowercase everywhere a machine
reads it — the command, packages, paths, configuration keys and values,
the Gitea check context. Environment variables keep their convention and
are uppercase throughout.

Every configuration file is still found under its pre-rename name
(`ConfigFiles`): `.gittally.yml` at the repository root, in a build
worktree and as committed on a branch, `.git/gittally/.gittally.yml` for
the machine layer. The current name wins where both exist, and the old
file is then ignored rather than merged — two files side by side are a
half-done rename, not a layering. Without the fallback an installation
that updated without renaming would not fail: a configuration that is
not found leaves every setting at its default, so it would come up
looking healthy while having forgotten its credentials and its builds.

`docs/werkator-migrationsplan.md` lists what the fallback does not
cover and has to be moved by hand — above all the state directory
`.git/werkator/`, which holds the build history, the control token and
the worktrees, and has no fallback of its own.

`docs/migration-from-legacy.md` is deleted with this: it mapped the
legacy script's environment variables, and every host it addressed has
long since moved to the YAML configuration.

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

CLAUDE.md

@@ -1,4 +1,4 @@
-# werkator — Claude Code Instructions
+# Werkator — Claude Code Instructions
 
 @AGENTS.md