Present the new-session sheet instantly; load branches behind it

[jamesrochabrun]

Problem

Users reported the start-thread sheet (sidebar pencil button) taking seconds to appear. Presentation was blocked on await loadBranches(), whose fetchAndGetDefaultRemoteBaseBranch runs a network git fetch --all — multi-second on slow networks and worst-case a DNS/connect timeout per unreachable remote. The Fork flow had the same await-before-present pattern.

Fix

• preselectRepository is now synchronous (repository match only). The pencil flow presents the sheet immediately and calls the new beginBranchLoad(), which sets isLoadingBranches/isLoadingCurrentBranch synchronously (no empty-picker flash on the first frame) and runs loadBranches() behind the sheet's existing loading state.
• configureForFork is synchronous too, seeding the form first and loading branches with the new applyDefaultBaseBranch: false mode — the late fetch fills the picker but can no longer overwrite fork's deliberate baseBranch = nil (base = current HEAD) or the seeded branch name.
• loadBranches is generation-guarded: if the user switches repositories while a slow fetch is in flight, the superseded load discards its results instead of stomping the newer repo's state.

Note: in worktree mode, launching before the fetch settles now bases the worktree on current HEAD (the picker shows "Loading…" during that window). Previously that window was spent staring at a frozen click before the sheet appeared.

Test-infra repairs (found while validating)

The app-target test bundle (app/AgentHubTests) had rotted unnoticed — nothing gates it headless:

• The shared AgentHub scheme had no testables, so xcodebuild … -scheme AgentHub test (the command in CLAUDE.md) errored. Added the AgentHubTests testable reference.
• Restored missing AgentHubSessionGraph / AgentHubCLIKit imports in five test files that no longer compiled.
• Heads-up: running these app-hosted tests headless additionally needs ad-hoc signing overrides (CODE_SIGN_IDENTITY=- CODE_SIGN_STYLE=Manual DEVELOPMENT_TEAM=) because the homebrew libgit2 dylib's Team ID doesn't match a dev-signed test host.

Testing

• New MultiSessionLaunchBranchLoadTests (headless core gate) pin the contract: fork seeds synchronously, the load preserves the seeded base, and the default flow still applies a base branch. 2/2 passed.
• Targeted package suites (preselection/resolver/context/prompt): 23/23 passed.
• App-target suites (fork, manual naming, AI naming, reset) with the signing override: 21/21 passed.
• Full gate ./scripts/test.sh: all fast packages ✓; the AgentHubCore suite reported failures only in the documented timing-flaky families (DiffAvailabilityService, SessionNameRequestMonitor, WorktreeDeletionRequestMonitor, WorktreeLaunchRequestMonitor, GitHubCIFailureNotifier, workspace-attach suites — see TestQuarantine.md/Re-enable 18 quarantined AgentHubCore tests (headless failures + hangs) #380), none intersecting this diff.

🤖 Generated with Claude Code