feat(memory): implement DC-MEM-006 local issue system client

[radoxtech]

Summary

• Implement IIssueClient backend-agnostic interface with full CRUD, list, search, and hierarchy operations for local issue management
• Add SQLite migration 010 with issues table, FTS5 virtual table, and write-through sync triggers
• Create IssueRepository implementing IIssueClient with parameterized queries, recursive CTE for descendant traversal, and FTS5 full-text search via bm25() scoring
• Add Zod schemas for Issue, IssueStatus, IssuePriority, input types, and filters
• 30 new tests covering all acceptance criteria

Fixes #95

Acceptance Criteria

• IIssueClient abstraction is backend-agnostic and testable via mocks
• SQLite adapter supports full CRUD + list + FTS5 search coverage
• Epic parent-child relationships supported via foreign keys with parent_id
• All operations are local-only (no network dependency)
• No non-abstract direct SQL calls leak into other packages (@diricode/memory boundary enforced)
• Issue schema matches spec: id, title, description, status, priority, labels (JSON), parent_id, created_at, updated_at
• FTS5 virtual table covers title and description with write-through sync

Test Results

• pnpm lint — 0 errors
• pnpm typecheck — 0 errors
• pnpm build — success
• pnpm --filter @diricode/memory test — 143/143 passed