Fix bd-l0r: Skip export after ZFC import to preserve JSONL source of truth

## Problem
When bd sync detected stale DB (>50% divergence), it would import JSONL to fix the DB,
but then immediately export the DB back to JSONL. This caused the stale DB to overwrite
the JSONL after a git pull, undoing cleanup work.

Example scenario:
1. Clone has 688 stale issues in DB (628 closed)
2. git pull brings JSONL with 62 issues (cleanup applied)
3. bd sync detects 1009.7% divergence and imports JSONL (DB → 62 issues) ✓
4. bd sync exports DB to JSONL (JSONL still 62 issues) ✓
5. But this marks JSONL as "changed" and commits/pushes it ✗

## Solution
After ZFC (JSONL First Consistency) import, set skipExport flag to prevent the export step.
JSONL is the source of truth after import - DB should sync to match, not export back.

## Changes
- cmd/bd/sync.go: Add skipExport flag, set it after ZFC import
- cmd/bd/sync.go: Wrap export logic in `if !skipExport` block
- CHANGELOG.md: Update ZFC entry with accurate description
- cmd/bd/sync_test.go: Add TestZFCSkipsExportAfterImport

Fixes #bd-l0r

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

CHANGELOG.md

@@ -83,10 +83,11 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
   - Now always uses content hash for reliable comparison
   - Prevents bd sync from overwriting pulled JSONL and resurrecting deleted issues
 
-- **ZFC (JSONL First Consistency)**: Enforce source of truth semantics (bd-l0r, 1ba068f, 2e4171a, 949ab42)
-  - bd sync now unconditionally imports JSONL first (source of truth)
-  - Simpler JSONL → DB → JSONL flow ensures consistency
-  - Prevents exporting when DB has significantly more issues than JSONL
+- **ZFC (JSONL First Consistency)**: Fix stale DB overwriting JSONL on sync (bd-l0r, 1ba068f, 2e4171a, 949ab42)
+  - bd sync now detects stale DB (>50% divergence from JSONL) and imports first
+  - After ZFC import, skips export to prevent overwriting JSONL source of truth
+  - Fixes bug where DB with 688 issues would overwrite JSONL with 62 issues after pull
+  - JSONL is source of truth after git pull - DB syncs to match, not vice versa
   - Preserves local uncommitted changes while catching stale DB scenarios
 
 - **Merge Conflict Semantics**: Improved resolution policies (bd-pq5k, d4f9a05)