M28-A: Sub-agent definition format and parser

[bug-ops]

Parent: #709

Summary

Implement the sub-agent definition format (markdown with TOML frontmatter) and parser. Reuse the +++ delimiter pattern from zeph-skills.

Scope

• SubAgentDef struct with fields: name, description, model, tools (ToolPolicy), permissions (SubAgentPermissions), skills (SkillFilter), system_prompt
• ToolPolicy enum: AllowList / DenyList / InheritAll
• SubAgentPermissions: secrets, max_turns, background, timeout_secs
• SkillFilter: include/exclude glob patterns
• TOML frontmatter parser (reuse pattern from zeph-skills loader)
• Definition loader scanning .zeph/agents/ and ~/.config/zeph/agents/

Acceptance Criteria

• SubAgentDef parses valid markdown+TOML definition files
• Invalid definitions produce clear error messages
• Loader discovers definitions from project and user directories
• Unit tests for parser edge cases (missing fields, defaults)
• serde(default) for optional fields with sensible defaults (max_turns=50, timeout_secs=300)