fix(ios): preserve authoritative task chat during navigation hydration

[CompleteDotTech]

Summary

• return the canonical task-chat thread when project-aware navigation is still hydrating
• keep terminal navigation failures as nil results
• add concurrency assertions for one thread, one session fetch, repaired links, and deferred navigation
• remove the now-stale quarantine entry for the repaired XCTest

Bead

• cave-q9nwj

Verification

• pnpm test:mobile — 98/98 files passed
• focused iOS contract, project-generation, CI, and XCTest-summary checks passed
• git diff --check

Native XCTest/build could not run in WSL because Xcode, xcodebuild, swift, swiftc, and xcrun are unavailable.