feat: dynamic multi-agent graph (replace hardcoded pipelines)

[alpibrupa]

Problem

multi_agent.lex has three hardcoded pipelines (build→test, build→spec→test+review). There is no way to compose arbitrary agent graphs at runtime — e.g. explore + plan in parallel, then hand results to build.

Design

Replace hardcoded pipelines with a small graph DSL:

type Node = AgentNode(AgentDef) | ParallelNode(List[Node]) | SequenceNode(List[Node])

fn run_graph(root: Node, input: Str, provider: ProviderTag) -> [concurrent, io, ...] GraphResult

This keeps the existing three presets as named constructors:

fn impl_then_test() -> Node { SequenceNode([AgentNode(build_agent()), AgentNode(test_agent())]) }
fn impl_then_spec_then_test() -> Node { ... }

And allows new compositions without touching the core runner.

The TUI --multi flag should accept a pipeline name or eventually a graph spec string.

Acceptance

• Existing three presets produce identical behavior
• New graph compositions can be expressed without modifying multi_agent.lex
• lex check --strict src/ passes