feat: Plugin coexistence — `apm pack --format plugin`, `apm init --plugin`, devDependencies

[danielmeppiel]

Summary

Implement plugin coexistence support per WIP/plugin-coexistence-spec-v2.md. This makes APM the supply chain layer FOR the plugin ecosystem by enabling APM-authored packages to be exported as valid standalone Copilot CLI / Claude Code plugin directories.

Key Changes

Phase 1: apm pack --format plugin

• New plugin_exporter.py module transforms .apm/ layout → plugin-native directory
• Maps agents, skills, prompts→commands, instructions, hooks, MCP configs
• Generates output plugin.json (from existing or synthesized from apm.yml)
• Flattens dependency components with lockfile-order-wins collision model
• Filters out devDependencies from output
• Output is a valid standalone plugin — no APM runtime needed to consume

Phase 2: apm init --plugin

• New --plugin flag creates plugin.json + apm.yml (2 files, nothing else)
• Name validation per plugin spec (kebab-case, max 64 chars)

Phase 3: devDependencies full support

• APMPackage model: dev_dependencies field
• apm install --dev flag
• Resolver + lockfile is_dev tracking
• Pack excludes dev deps from output

Phase 4: Pure plugin consumption polish

• Root-level discovery patterns for plugin-native repos
• Verify end-to-end install of plugin.json-only repos

Design Decisions

• .apm/ layout retained — no deprecation, no migration
• apm pack default behavior unchanged — plugin export is behind --format plugin
• No breaking changes — all new functionality behind flags
• Zero vendor lock-in: plugin output needs no APM runtime

Spec

See WIP/plugin-coexistence-spec-v2.md for full specification.