feat(interop): add MCP and A2A adapters

[douglas-grishen]

What changed

• Adds executable MCP Streamable HTTP client/server adapters and governed remote-tool import.
• Adds A2A v1 HTTP+JSON client/provider/server support, SSE streaming, task subscriptions and workflow-backed task persistence.
• Adds a framework-neutral Fetch mount helper for MCP and A2A plus public host-integration guidance.
• Extends workflow lifecycle states for A2A rejection, input-required and authentication-required outcomes.

Why

Makes AgentPlat interoperable with external agent and tool ecosystems through open protocols while preserving host-owned authentication, tenant resolution and authorization.

Validation

• pnpm --filter @agentplat/mcp build && pnpm --filter @agentplat/mcp test
• pnpm --filter @agentplat/a2a build && pnpm --filter @agentplat/a2a test
• pnpm --filter @agentplat/interop build && pnpm --filter @agentplat/interop test

[douglas-grishen]

Backlog cleanup decision: CONTINUE / REWORK. This PR represents independent work and remains open, but it is not currently a merge candidate: review is required and the current branch/check state must be repaired before integration.

[douglas-grishen]

Final decision: RETAIN AS REBASE/REWORK SOURCE. MCP/A2A interoperability is not present in current main, so this is not superseded; however the branch is substantially behind and dirty. Rebase it onto current main, resolve API drift, then split the result into reviewable MCP and A2A increments.