Implement incremental JSONL export with dirty issue tracking

Optimize auto-flush by tracking which issues have changed instead of
exporting the entire database on every flush. For large projects with
1000+ issues, this provides significant performance improvements.

Changes:
- Add dirty_issues table to schema with issue_id and marked_at columns
- Implement dirty tracking functions in new dirty.go file:
  * MarkIssueDirty() - Mark single issue as needing export
  * MarkIssuesDirty() - Batch mark multiple issues efficiently
  * GetDirtyIssues() - Query which issues need export
  * ClearDirtyIssues() - Clear tracking after successful export
  * GetDirtyIssueCount() - Monitor dirty issue count
- Update all CRUD operations to mark affected issues as dirty:
  * CreateIssue, UpdateIssue, DeleteIssue
  * AddDependency, RemoveDependency (marks both issues)
  * AddLabel, RemoveLabel, AddEvent
- Modify export to support incremental mode:
  * Add --incremental flag to export only dirty issues
  * Used by auto-flush for performance
  * Full export still available without flag
- Add Storage interface methods for dirty tracking

Performance impact: With incremental export, large databases only write
changed issues instead of regenerating entire JSONL file on every
auto-flush.

Closes bd-39

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

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

internal/storage/sqlite/schema.go

@@ -71,6 +71,16 @@ CREATE TABLE IF NOT EXISTS config (
     value TEXT NOT NULL
 );
 
+-- Dirty issues table (for incremental JSONL export)
+-- Tracks which issues have changed since last export
+CREATE TABLE IF NOT EXISTS dirty_issues (
+    issue_id TEXT PRIMARY KEY,
+    marked_at DATETIME NOT NULL DEFAULT CURRENT_TIMESTAMP,
+    FOREIGN KEY (issue_id) REFERENCES issues(id) ON DELETE CASCADE
+);
+
+CREATE INDEX IF NOT EXISTS idx_dirty_issues_marked_at ON dirty_issues(marked_at);
+
 -- Ready work view
 CREATE VIEW IF NOT EXISTS ready_issues AS
 SELECT i.*