POC: Add plugin discovery and execution system

[chasdr]

Summary

Adds a plugin discovery and execution system to the DR CLI, allowing external executables (e.g., dr-agentassist) to be discovered and invoked as subcommands.

Related PR

• Agent-Assist Plugin: datarobot/dr-agent-cli#63

Changes

New Files

File	Description
internal/plugin/types.go	PluginManifest, PluginCommand, DiscoveredPlugin structs
internal/plugin/discover.go	PATH scanning, manifest fetching with 2s timeout
internal/plugin/exec.go	Subprocess execution with signal forwarding
PLUGIN_POC.md	Installation and usage documentation

Modified Files

File	Description
cmd/root.go	Plugin registration and collision detection

How It Works

1. Discovery: On startup, scans .datarobot/cli/bin/ and PATH for dr-* executables
2. Manifest: Calls each plugin with --dr-plugin-manifest to get JSON metadata
3. Registration: Adds plugins as subcommands under "Plugin Commands" group
4. Execution: Forwards stdin/stdout/stderr and signals to plugin subprocess

Local Testing Instructions

Step 1: Build the CLI

# Clone and checkout this branch
git clone https://github.com/chasdr/cli.git
cd cli
git checkout chas/plugin-poc

# Build
task build

# Verify build
./dist/dr --version

Step 2: Install the Agent-Assist Plugin

# Clone the agent-assist repo
git clone https://github.com/datarobot/dr-agent-cli.git
cd dr-agent-cli
git checkout chas/plugin-poc

# Install as a tool (adds dr-agentassist to PATH)
cd mdb
uv tool install --force --editable .

# Verify installation
which dr-agentassist
dr-agentassist --dr-plugin-manifest

Step 3: Test Plugin Discovery

# From the cli directory
cd /path/to/cli

# Check help shows plugin
./dist/dr --help
# Should show:
# Plugin Commands:
#   agentassist   AI agent design, coding, and deployment assistant

# Run the plugin via CLI
./dist/dr agentassist --help

Step 4: Verify Signal Handling

# Start the plugin
./dist/dr agentassist

# Press Ctrl+C - should propagate to plugin and exit cleanly

Plugin Manifest Format

Plugins must respond to --dr-plugin-manifest with JSON:

{
  "name": "agentassist",
  "version": "0.1.0",
  "description": "AI agent design, coding, and deployment assistant",
  "commands": [{
    "name": "agentassist",
    "description": "Interactive agent development assistant"
  }]
}