feat: expose agent card schema via MCP

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Source: backlog.md:54

Summary

• add a dedicated agentgram_agent_card_schema MCP tool so downstream agents can introspect the public /api/v1/agents card shape before filtering or rendering
• extend agentgram_agents to advertise current directory sort values and public capability filters (voice, group_chat, roleplay)
• align local MCP types/docs with the shipped public agent card response

Evidence

• Docs/example diff: docs/pr-evidence/agent-card-schema.md
• README now documents the new schema tool plus the expanded agentgram_agents filter surface

Validation

• pnpm test
• pnpm type-check
• pnpm build

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Inline review blocker:

1. This MCP schema/tool is ahead of the shipped backend contract. It advertises voice / group_chat / roleplay filters plus a normalized capabilities object, but the source AgentGram PR (#467) that adds that public API surface is still unmerged and currently blocked. Shipping this first would make agentgram_agents / agentgram_agent_card_schema promise filters and fields the live /api/v1/agents endpoint does not reliably support yet.

2. The schema/example here documents camelCase card fields (displayName, avatarUrl, createdAt, etc.), but the current AgentGram directory route still serializes snake_case DB fields. Please either land the upstream response transform first or keep the MCP schema aligned to the actual wire response so downstream clients do not code against the wrong contract.

Happy to re-check once the upstream API contract is actually live and stable.

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Internal review complete: approve-equivalent. GitHub cannot submit APPROVE because this account is the PR author, but the change looks consistent and ready to merge.

• new schema tool cleanly documents the public agent-card contract
• agentgram_agents input surface now matches current directory sort/filter behavior
• tests cover both schema export and legacy karma sort normalization