fix(sync): ensure sync-branch worktree exists on fresh clone

[jdelfino]

Summary

On fresh clone, findJSONLPath() would silently fall back to main's stale JSONL because the sync-branch worktree didn't exist yet. The worktree was only created on first daemon sync (write operation), but import operations (reads) would use the stale data.

This caused sync failures when containers were rebuilt - issues accumulated on beads-sync branch were not imported after rebuild because main's stale JSONL was used instead. The data still existed on beads-sync, but the local database was populated with outdated state.

The Fix

1. Add EnsureWorktree() in internal/syncbranch/worktree.go - creates the sync-branch worktree if configured but doesn't exist
2. Call EnsureWorktree() from ensureStoreActive() in direct_mode.go - guarantees worktree exists before any JSONL operations
3. Add debug logging when getWorktreeJSONLPath() falls back (should now be rare)

The EnsureWorktree() function is idempotent - safe to call multiple times. The underlying CreateBeadsWorktree also performs health checks and auto-repairs unhealthy worktrees.

Root Cause Analysis

The issue was in the read path:

• findJSONLPath() calls getWorktreeJSONLPath()
• If worktree doesn't exist, it returns "" causing fallback to main's JSONL
• On fresh clone, worktree doesn't exist yet
• Post-checkout hook runs bd sync --import-only which uses the stale path

The write path worked correctly - daemon creates worktree before committing. But reads happened before writes on fresh clone.

Tests

• Worktree not created when sync-branch not configured
• Worktree created when sync-branch configured via env
• Worktree created when remote branch exists
• Idempotent behavior on repeated calls
• Fresh clone scenario end-to-end test - simulates the exact broken scenario

Test plan

• Run go test ./internal/syncbranch/... -run TestEnsureWorktree
• Run go test ./internal/syncbranch/... -run TestFreshCloneScenario
• Run full test suite: go test ./...
• Manual test: clone repo with sync-branch configured, verify worktree created on first bd command

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