Implement config show CLI with Layer Visibility

## Overview

Implement `config show` CLI command that displays the currently applied configuration with visibility into which loader (file, env, etcd) each value originates from.

## Requirements

- Display merged configuration values
- Show source layer for each config field (file, environment, etcd)
- Support filtering by config section
- Output in human-readable format (table/tree) and machine-readable format (JSON/YAML)

## Example Output

```
[database]
host: localhost (source: file)
port: 5432 (source: etcd)
pool_size: 10 (source: env)

[etcd]
addr: 127.0.0.1:2379 (source: file)
```

## Acceptance Criteria

- [ ] Shows all config values with their source layer
- [ ] Supports --format option (table, json, yaml)
- [ ] Supports --section option to filter specific sections
- [ ] Works for all components (manager, agent, storage, webserver, app-proxy)

JIRA Issue: BA-3473