feat(reporting): add comprehensive fleet-wide reporting system

Implements fleet-wide reporting with multiple output formats.

Features:
--------
- Generate reports in 3 formats: Markdown, JSON, HTML
- Fleet-wide metrics and summaries
- Bot execution tracking with durations
- Findings aggregation by severity and bot
- Tier performance analytics
- Overall fleet health scoring (0-100)

Report Contents:
---------------
1. **Fleet Summary**
   - Total bots (registered/completed/in-progress/pending)
   - Finding counts by severity
   - Overall health score

2. **Bot Executions**
   - Individual bot status
   - Findings and error counts
   - Execution durations

3. **Tier Performance**
   - Bots per tier
   - Completion rates
   - Average durations
   - Findings distribution

4. **Findings Analysis**
   - By severity (Error/Warning/Info/Suggestion)
   - By bot (which bot found what)

Health Scoring:
--------------
- 50% from completion rate (bots completed / total)
- 50% from severity penalties (errors/warnings impact)
- Range: 0-100 (higher = better)

CLI Integration:
---------------
Updated `fleet report` command to generate reports:
```bash
fleet report                    # Markdown to stdout
fleet report --format json      # JSON to stdout
fleet report --output report.md # Save to file
fleet report --format html -o report.html
```

Implementation:
--------------
- shared-context/src/reporting.rs (400+ lines)
- Comprehensive test suite (4 tests)
- fleet-cli integration

Build Status: ✅ All components compile successfully

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

Author: Jonathan D.A. Jewell