Release v0.7.1 — Windows binary + structural polish · shhac/git-wt

Patch release. Adds a Windows binary plus a round of structural polish; no user-facing behaviour changes for the existing four platforms.

Changes

Windows binary now ships. The wrapper-protocol (fd<N> / N>&1 1>&2) is POSIX-only by design — the Windows binary always operates in bare mode. Use it from PowerShell with stdout capture:
```
function gwt {
    $target = & git-wt.exe @args
    if ($target) { Set-Location $target }
}
```
Or from cmd.exe via doskey:
```
doskey gwt=for /f "delims=" %i in ('git-wt.exe $*') do cd /d "%i"
```
Git Bash / MSYS2 / WSL2 users continue to use the full POSIX wrapper from git-wt alias gwt.
Internal refactor (no behaviour change): extract loadWorktrees + moveCursor to dedup; split column-layout helpers into columns.go; split e2e harness/fixtures; small complexity wins in printList / executeRm.
Test coverage: ~30 new unit tests covering the in-progress-op marker walk (now extracted as inProgressOp), copyConfigs error paths, and collectCleanTargets (now with DI seams for branchExists and listGone).
Lint / format: .golangci.yml standardised with sibling repos (agent-sql, lin, etc.); CI workflow renamed test.yml → ci.yml and split into unit (build + lint + short tests) and e2e (full suite, ubuntu + macos matrix).

brew install shhac/tap/git-wt          # new
brew upgrade shhac/tap/git-wt          # from 0.7.0

Pre-built binaries: darwin-arm64, darwin-amd64, linux-amd64, linux-arm64, windows-amd64.

shasum -a 256 -c checksums-sha256.txt