fix: route git through project helper containers

[numnx]

Summary

• Routes normal backend Git execution through containerized alpine/git helpers and keys warm helpers by project Git common directory plus UID/GID so repo-local worktrees share one helper.
• Removes host Git from onboarding readiness and dependency installation while preserving existing installer mode ids for API compatibility.
• Reworks workspace patch export/materialization to use direct Git argv calls and NUL pathspec files instead of shell scripts.
• Updates dashboard copy, tests, and docs to describe Docker-only readiness and containerized helper Git.

Root Cause

Helper reuse was effectively scoped to task worktree paths, and several app-owned Git paths still assumed host Git or shell-driven Git execution. On Windows this surfaced as shell spawn failures and fragile path handling when patch materialization used host-shaped temp paths.

Validation

pnpm exec vitest run tests/backend/shared/subprocess/command-runner.test.ts tests/backend/infrastructure/providers/cli/workspace-artifact-service.test.ts tests/backend/infrastructure/providers/cli/workspace-manager.test.ts tests/backend/services/onboarding-readiness-service.test.ts tests/backend/services/onboarding-dependency-installer-service.test.ts tests/backend/app/lifecycle/dashboard-lifecycle-service.test.ts tests/backend/server/dashboard-routes-error.test.ts tests/dashboard/onboarding/onboarding-installation-step.test.tsx tests/dashboard/onboarding/onboarding-flow.test.tsx --pool=threads
pnpm exec vitest run tests/backend/infrastructure/providers/cli/provider-runner.test.ts tests/backend/domain/sprint/ci/feature-pr-gate.test.ts tests/backend/sprint/watch-loop-core.test.ts --pool=threads
pnpm run test:backend
pnpm run lint
pnpm run build