Refactor worktree helper flows (#540)

## Summary
- refactor internal/worktree helper flows around smaller git and
submodule operations
- clean up worktree tests to use direct testify assertions and stronger
helper coverage
- preserve nested local submodule initialization by restoring the
two-pass protocol detection

## Validation
- go fmt ./...
- go test ./...
- go vet ./...
- go test -tags=integration ./internal/worktree/...

---------

Co-authored-by: Wes McKinney <wesmckinn+git@gmail.com>
Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: 3 people

CLAUDE.md

@@ -8,6 +8,10 @@ roborev is an automatic code review daemon for git commits. It runs locally, tri
 
 When a task involves multiple steps (e.g., implement + commit + PR), complete ALL steps in sequence without stopping. If creating a branch, committing, and opening a PR, finish the entire chain.
 
+### Handling review findings
+
+When the user pastes review findings directly in the conversation (verdicts, severities, file paths, suggested fixes), treat them as direct instructions and fix the code normally. Do NOT invoke the `/roborev-fix` skill — that skill is only for discovering and fetching open reviews from the daemon, not for processing findings already present in the prompt.
+
 ## Go Development
 
 After making any Go code changes, always run `go fmt ./...` and `go vet ./...` before committing. Stage ALL resulting changes, including formatting-only files.

internal/worktree/uid_unix_test.go

@@ -0,0 +1,7 @@
+//go:build !windows
+
+package worktree
+
+import "os"
+
+func isRoot() bool { return os.Getuid() == 0 }

internal/worktree/uid_windows_test.go

@@ -0,0 +1,5 @@
+//go:build windows
+
+package worktree
+
+func isRoot() bool { return false }