test(project): use effect service fixtures

[kitlangton]

Summary

• migrate project tests from local Project service callback helper to Project.use service accessors
• use AppFileSystem.use for Effect-native file existence and favicon writes
• keep git temp dir, worktree, bare repo, and shell-driven git setup semantics unchanged

Verification

• bunx prettier --write packages/opencode/test/project/project.test.ts
• bun typecheck (packages/opencode)
• bun test --timeout 5000 test/project/project.test.ts (packages/opencode)
• bunx oxlint packages/opencode/test/project/project.test.ts (warnings only: pre-existing unsafe assertions in mock/test IDs)
• test name diff before/after: no diff