feat: Native OpenCode integration

[danielmeppiel]

Summary

Add native OpenCode as an opt-in integration target for APM.

When .opencode/ directory exists in a project, apm install deploys:

• Agents → .opencode/agents/<name>.md
• Commands → .opencode/commands/<name>.md (from .prompt.md files)
• Skills → .opencode/skills/<name>/SKILL.md
• MCP → opencode.json (new OpenCodeClientAdapter)

OpenCode reads AGENTS.md natively, so apm compile already works for instructions.

Background

@timvw opened PR #257 with initial OpenCode support. That PR conflicts with the architecture refactoring done in PR #301 (Cursor integration), which introduced TargetProfile + KNOWN_TARGETS + data-driven prefix resolution. This issue tracks a proper implementation using the current architecture.

Acceptance Criteria

• opencode TargetProfile in KNOWN_TARGETS
• Target auto-detection for .opencode/ folder
• Agent, command, skill integration (opt-in)
• OpenCodeClientAdapter for opencode.json MCP format
• Pack/compile support (--target opencode)
• Install/uninstall wiring
• Unit tests
• Documentation updates