[Fix #30] docs(agents): align agent definitions with v2 plugin schemas

[diegohb]

Summary

• Align all ten agent definitions in agents/ with the v2 plugin system.
• Architect: routing notes and packaging checks use the v2 plugins registration key; the templates pointer resolves from agents/.
• Extension-auditor: inventories v2 targets and judges v2 conformance.
• MCP-integrator, agent-designer, command-crafter, and skill-creator: teach v2 schemas and frontmatter knobs.
• Packager and publisher: emit v2-native plugins-key registration snippets with consumer instructions.
• Tool-builder: pivots to plugin-registered tools while documenting the v1 tool-file port path.
• Plugin-engineer: Effect-first v2 authoring guidance and the hook-family map, keeping the hooks-never-throw advisory.

Why

Issue #30: the agent definitions still taught the previous registration flow, so guidance and generated snippets used stale keys and paths. Bringing every agent up to the v2 schemas means authored plugins, tools, and commands register correctly out of the box.

Testing

• Docs-only change across 10 files in agents/; no runtime code touched.
• Registration snippets and packaging checks in the docs now reference the v2 plugins registration key.
• Cross-checked agent files for consistent v2 schema and frontmatter guidance.