feat: add unified fleet CLI for gitbot-fleet coordination

Implements Task #26 from Opus recovery session.

New fleet-cli binary provides single entry point for all fleet operations:

Commands:
- fleet run [--bots BOTS] [--continue-on-error] - Execute fleet on repo
- fleet status [--detailed] - Show fleet execution status
- fleet list [--tier N] - List available bots (0=Engine, 1=Verifier, 2=Finisher, 3=Executor)
- fleet findings [--bot BOT] [--severity LEVEL] [--json] - Show findings with filters
- fleet report [--output FILE] [--format FORMAT] - Generate reports (markdown/json/html)

Features:
- Colored terminal output for readability
- Tier-based bot filtering
- Error recovery (--continue-on-error flag)
- Async execution with tokio
- Verbose logging mode

Architecture:
- Uses gitbot-shared-context for fleet coordination
- Orchestrates cross-bot communication
- Publishes findings to shared context
- Supports multi-bot workflows

Build Status: ✅ Compiles successfully with only warnings

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

Author: Jonathan D.A. Jewell