feat(plugin): help integration with --help-oneliner protocol (Phase 2)

[gwpl]

Summary

Phase 2 of external plugin architecture (Issue #188). Builds on Phase 1 (#189).

Adds plugin discovery and help integration:

• Plugins appear in gog --help under a new "Plugins:" section
• Uses --help-oneliner protocol for plugin descriptions

Features

Plugin Discovery

• Scans PATH for all gog-* binaries
• Lazy discovery: only when help is requested
• Deduplicates: first PATH occurrence wins

--help-oneliner Protocol

• Plugins respond to --help-oneliner with single line (≤80 chars)
• 100ms timeout keeps help responsive
• Plugins without protocol support show no description

Help Integration

• "Plugins:" section after "Commands:"
• Aligned descriptions for readability
• Colorized like built-in commands

Example Output

Commands:
  gmail <command> [flags]
    Gmail
  ...

Plugins:
  docs headings    List document headings with hierarchy and URLs
  hello            Say hello (example plugin)
  sheets formulas  Extract and analyze spreadsheet formulas

Example Plugin

#!/bin/bash
if [[ "$1" == "--help-oneliner" ]]; then
    echo "List document headings with hierarchy and URLs"
    exit 0
fi
# Plugin implementation...

Changes

• internal/cmd/external.go - DiscoverExternalPlugins(), FetchOneliners()
• internal/cmd/external_test.go - Tests for grouping and CommandName()
• internal/cmd/help_printer.go - injectExternalPlugins(), colorization

Test plan

• Unit tests for plugin grouping
• Unit tests for CommandName()
• Manual test with shell script plugins
• Verified help output formatting
• Verified colorization works

Dependencies

Depends on Phase 1: #189

---

Part of #188

Generated with Claude Code

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