chore(release): v6.3.19 - Dense Reranker, CLI Tools & Issue Workflow

## Documentation Updates
- Update all version references to v6.3.19
- Add Dense + Reranker search documentation
- Add OpenCode AI CLI tool integration docs
- Add Issue workflow (plan → queue → execute) with Codex recommendation
- Update CHANGELOG with complete v6.3.19 release notes

## Features
- Cross-Encoder reranking for improved search relevance
- OpenCode CLI tool support
- Issue multi-queue parallel execution
- Service architecture improvements (cache-manager, preload-service)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

.claude/commands/issue/execute.md

@@ -33,6 +33,8 @@ Minimal orchestrator that dispatches **solution IDs** to executors. Each executo
 **Executor & Dry-run**: Selected via interactive prompt (AskUserQuestion)
 **Worktree**: Creates isolated git worktrees for each parallel executor
 
+**⭐ Recommended Executor**: **Codex** - Best for long-running autonomous work (2hr timeout), supports background execution and full write access
+
 **Worktree Options**:
 - `--worktree` - Create a new worktree with timestamp-based name
 - `--worktree <existing-path>` - Resume in an existing worktree (for recovery/continuation)