Bound Xcode build output capture

[jamesrochabrun]

Summary

• Replace the managed Xcode build-cache cleanup feature with a smaller memory-focused optimization.
• Bound retained stdout and stderr from AgentHub-started Xcode builds to 1 MiB per stream while still draining both pipes during the build.
• Add a focused test that verifies the build-output accumulator keeps only the newest bytes.

User impact

• AgentHub no longer adds SwiftPM package cache or cloned source package path overrides in this PR.
• Build and run behavior keeps the existing DerivedData behavior from main.
• Long xcodebuild output can no longer grow AgentHub's in-memory output buffer without bound; failure reporting still uses the retained output tail.

Review notes

• The prior cache-management approach was removed instead of expanding it to cover scheme detection and per-workspace cloned package checkouts.
• This keeps the optimization less invasive and avoids rerouting user toolchain/package cache paths.

Validation

• Passed: git diff --check main...HEAD
• Attempted: swift test --package-path app/modules/AgentHubCore --filter BuildHelperTests
• Blocked by existing dependency issue: CodeEditSymbols fails to compile because Bundle.module is unavailable while Symbols.xcassets is not declared as a package resource.