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>

docs/adrs/0001-2026-06-09.test-framework.md

@@ -10,7 +10,7 @@
 
 ## Context and Problem Statement
 
-werkator is a greenfield Kotlin/Spring Boot project.
+Werkator is a greenfield Kotlin/Spring Boot project.
 A test framework must be chosen before writing any tests.
 
 The framework shapes how tests are structured, how readable they are, and how well they integrate with the Spring Boot test slice infrastructure.