Ports the SKILL.md/frontmatter pattern from skills-claude (fork of
anthropics/skills) to define user-facing "niches" (legal, financial,
business-capability) that each group one or more AGENT_TOOL_REGISTRY
entries. NicheRegistryService loads and validates src/agent-niches/
definitions/*.md at boot, failing loudly on unknown tool references so a
niche can never silently grant access to an undeclared capability. This
is the data model for the planned dashboard where memberships access
Hermes agents scoped by niche instead of by raw tool name.
