feat: make bootstrap/run.lex a general-purpose multi-phase pipeline

[alpibrupa]

Problem

src/bootstrap/run.lex is hardcoded to implement list.zip as a demo. It cannot be pointed at a real task.

Fix

Refactor run.lex into a parameterized pipeline:

type Phase = Build | Spec | Test | Review | Custom(AgentDef)
type Pipeline = { phases: List[Phase], task: Str, provider: ProviderTag }

fn run_pipeline(p: Pipeline) -> [io, concurrent, ...] PipelineResult

The existing four-phase demo becomes:

fn demo_pipeline() -> Pipeline {
  { phases: [Build, Spec, Test, Review], task: "implement std.list.zip", provider: Anthropic }
}

The TUI --multi flag should be able to run a named pipeline or accept --pipeline build,spec,test.

Acceptance

• run_pipeline accepts arbitrary task strings
• demo_pipeline() reproduces existing behavior
• lex check --strict src/ passes