Supports custom subagents overview

The agent platform can load user-defined subagent definitions from disk (parallel to a Skill's SKILL.md). The on-disk format is recorded on `AgentPlatformImpl.subagentDefinitionFormat` (e.g. `claude-code-md` for Claude Code's `.claude/agents/<name>.md` Markdown-with-YAML-frontmatter format). Distinct from `capability:supports-subagent-dispatch` — a runtime can dispatch built-in subagents without supporting custom definitions, and a platform can declare custom-subagent loading even on top of a runtime that uses a non-task-tool dispatch mechanism. References: - Anthropic Claude Code custom subagents: https://docs.anthropic.com/en/docs/claude-code/sub-agents ('.claude/agents/<name>.md' project-scope; '~/.claude/agents/<name>.md' user-scope.)