feat: add feature-lead engine

[Copilot]

Adds the feature-lead engine — a two-step agentic workflow invoked by the orchestrator for each FeatureLead it produces. The engine breaks a feature into discrete tasks via a task-breaker agent, then drives a coder agent to implement each task against the feature's resolved contracts.

New types in orchestrator

• Task — discrete implementable unit (name, description, acceptance criteria, files)
• CoderOutput — coder agent response tied to a task
• FeatureLeadResult — aggregated result per lead (tasks + coder outputs)
• FeatureLeadRunner interface — decouples the orchestrator from the engine implementation; featurelead imports orchestrator, not the reverse

orchestrator.New — functional options (backward-compatible)

// existing callers unchanged; engine wired in optionally
orch := orchestrator.New(client, names, mcpURL, repoPath,
    orchestrator.WithFeatureLeadRunner(flEngine),
)

Result.FeatureLeadResults (omitempty) is populated only when a runner is configured.

New featurelead package

Engine implements FeatureLeadRunner with two steps:

1. Task breakdown — task-breaker agent decomposes the FeatureLead JSON into []Task
2. Task implementation — coder agent is invoked once per task, receiving the task spec plus the feature's resolved RequiredContracts and ProvidedContracts

main.go

Two new optional flags — --task-breaker-agent and --coder-agent — must be supplied together. When present, a featurelead.Engine is wired into the orchestrator.

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