Fix bd-160: Implement JSONL integrity validation and prevent export deduplication data loss

## Problem
Export deduplication feature broke when JSONL and export_hashes diverged
(e.g., after git pull/reset). This caused exports to skip issues that
weren't actually in the file, leading to silent data loss.

## Solution
1. JSONL integrity validation before every export
   - Store JSONL file hash after export
   - Validate hash before export, clear export_hashes if mismatch
   - Automatically recovers from git operations changing JSONL

2. Clear export_hashes on all imports
   - Prevents stale hashes from causing future export failures
   - Import operations invalidate export_hashes state

3. Add Storage interface methods:
   - GetJSONLFileHash/SetJSONLFileHash for integrity tracking
   - ClearAllExportHashes for recovery

## Tests Added
- TestJSONLIntegrityValidation: Unit tests for validation logic
- TestImportClearsExportHashes: Verifies imports clear hashes
- TestExportIntegrityAfterJSONLTruncation: Simulates git reset (would have caught bd-160)
- TestExportIntegrityAfterJSONLDeletion: Tests recovery from file deletion
- TestMultipleExportsStayConsistent: Tests repeated export integrity

## Follow-up
Created bd-179 epic for remaining integration test gaps (multi-repo sync,
daemon auto-sync, corruption recovery tests).

Closes bd-160

cmd/bd/export.go

@@ -2,6 +2,8 @@ package main
 
 import (
 	"context"
+	"crypto/sha256"
+	"encoding/hex"
 	"encoding/json"
 	"fmt"
 	"os"
@@ -273,6 +275,17 @@ Output to stdout by default, or use -o flag for file output.`,
 			// Clear auto-flush state since we just manually exported
 			// This cancels any pending auto-flush timer and marks DB as clean
 			clearAutoFlushState()
+			
+			// Store JSONL file hash for integrity validation (bd-160)
+			jsonlData, err := os.ReadFile(finalPath)
+			if err == nil {
+				hasher := sha256.New()
+				hasher.Write(jsonlData)
+				fileHash := hex.EncodeToString(hasher.Sum(nil))
+				if err := store.SetJSONLFileHash(ctx, fileHash); err != nil {
+					fmt.Fprintf(os.Stderr, "Warning: failed to update jsonl_file_hash: %v\n", err)
+				}
+			}
 		}
 
 		// If writing to file, atomically replace the target file