P1: Support multi-workflow MCP server from manifest/config

[Neftedollar]

Parent epic: #236

Problem

Current CLI flow is effectively single-workflow-first, which makes team deployments awkward.

Scope

• Add manifest-based startup (agentwf mcp serve --manifest <file>).
• Support multiple workflows with stable tool naming and collision checks.
• Document naming/versioning strategy for tool identifiers.

Acceptance Criteria

• One MCP process can expose multiple workflows.
• Tool names are deterministic and collision-safe.
• Existing single-workflow usage remains backward-compatible.
• Integration tests cover at least two workflows in one server instance.

Non-goals

• Dynamic hot-reload of workflow set at runtime.