Idea-intake API/MCP tool

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Problem: #4779's schema needs a real submission surface a customer can actually use.

Area: MCP / API

Proposal: Implement an MCP tool and/or API endpoint that accepts a freeform idea and produces a task-graph conforming to #4779's schema.

Deliverables:

• A working tool/endpoint validated against the Idea-intake bridge schema #4779 schema.

Acceptance criteria:

• Submitting each of F1's worked examples produces the expected task-graph.
• A malformed or empty submission returns a clear, actionable error rather than a silent failure.

Test scenarios:

• Schema-validation unit tests; a trivial-idea test; a complex-idea test; a malformed-input test.

Boundaries:

• Blocked on Idea-intake bridge schema #4779's approval.

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Part of #4778.

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JSO-3470