fix(service): find bundled KumoService helper from CLI in Contents/Helpers

[tom-cat-mao]

Summary

• Fix kumo service install failing with "KumoService executable was not found" when invoked through the bundled CLI (Kumo.app/Contents/Helpers/kumo), even though the app ships the helper at Kumo.app/Contents/MacOS/KumoService.
• Add the missing lookup candidate: from the CLI executable's directory, resolve the sibling Contents/MacOS/KumoService. It sits after the Bundle.main-derived candidates and before the working-directory fallbacks, so a bundled helper outranks stray files; existing candidates and order are unchanged.
• Extract the candidate list into a pure, testable helperExecutableCandidates(bundleURL:executableURL:workingDirectory:installedHelperURL:) and add two XCTest cases covering CLI-style and GUI-style resolution.

Root cause

The CLI ships in Contents/Helpers/ rather than MacOS/ to avoid colliding with the case-insensitive Contents/MacOS/Kumo GUI binary (documented in docs/interfaces/cli-agent-control.md). In the CLI process Bundle.main does not resolve to Kumo.app, so all bundle-derived candidates pointed inside Contents/Helpers/ and the helper's real location one directory over was never searched. GUI installs worked because Bundle.main = Kumo.app. This PR keeps the documented layout and only extends the lookup accordingly.

Verification

• swift build --target KumoCoreKit passes.
• Candidate resolution verified at runtime for both layouts: CLI-in-Helpers now finds Contents/MacOS/KumoService ahead of working-directory fallbacks; GUI resolution keeps Contents/MacOS/KumoService first.
• The new tests follow the repo's XCTest conventions but could not be executed locally: this environment has Command Line Tools only (no XCTest framework, and resource compilation requires Xcode's xcstringstool). Please run swift test --filter KumoServiceManagerTests on an Xcode-equipped machine.