feat(story): add StoryOrchestrator -- Builder->Evaluators->Arbitration->accept (Phase 7b)

A deterministic, poll-based watcher (internal/scheduler.StoryOrchestrator,
sibling to the Phase 5 Scheduler) that drives a story.Story through its
execution pipeline, rather than relying on an LLM agent to correctly
orchestrate its own fan-out via tool calls.

Mechanism: polling, not a handleRunResult hook. Every task the orchestrator
watches (a story's root/Builder task, 4 Evaluators, Arbitration) is
top-level (no ParentTaskID), and executor.Pool.handleRunResult only ever
lands a top-level task at READY or BLOCKED -- never COMPLETED directly, since
that transition normally requires a human/chatbot POST /api/tasks/{id}/accept
in a different package. A handleRunResult hook would never observe it;
polling doesn't care how/whether a task reached a given state.

Stages: Builder COMPLETED -> spawn 4 role-typed Evaluator tasks
(evaluator_quality/security/correctness/performance, DependsOn: [builder],
no ParentTaskID -- true DAG siblings, not delegated subtasks) + story ->
VALIDATING. Each Evaluator COMPLETED -> emit KindEvalVerdict (attached to
the story's ID, so one GET /api/stories/{id}/events call surfaces every
verdict). All 4 Evaluators COMPLETED -> spawn 1 Arbitration task
(role: planner, DependsOn: all 4 evaluator IDs). Arbitration COMPLETED ->
emit KindArbitrationDecided, story -> REVIEW_READY. POST
/api/stories/{id}/accept (mirrors handleAcceptTask) -> DONE, emits
KindHumanAccepted.

Fixes a gap caught before merging: since none of Builder/Evaluators/
Arbitration have a ParentTaskID, none of them auto-complete -- each would
otherwise need a separate manual /api/tasks/{id}/accept, meaning 6 human
clicks per story before ever reaching the intended single story-level gate.
StoryOrchestrator.autoAccept now transitions each of these specific tasks
READY->COMPLETED itself (via the same validated Store.UpdateTaskState path
acceptTask uses), scoped only to tasks already established as part of a
story's pipeline (root task, or role-matched dependents from
ensureEvaluators/ensureArbitration) -- never a blanket sweep of unrelated
READY tasks. This makes POST /api/stories/{id}/accept the system's only
required human touchpoint for the whole chain, matching the design goal
that story (not task/subtask) is the human-interaction atom.

Idempotency: structural for task-creation stages (ensureEvaluators/
ensureArbitration check ListDependents for already-existing role-matched
tasks before creating -- crash/restart-safe); story.Status=="VALIDATING"
gates the Arbitration->REVIEW_READY write (nothing further downstream to
check structurally there); an in-memory handledVerdicts set (mirrors
Scheduler.handled) dedupes per-evaluator KindEvalVerdict emission across
poll ticks, resetting harmlessly on restart.

Documented simplification: finalizeArbitration never parses the Arbitration
summary for approve/reject -- always routes to REVIEW_READY; NEEDS_FIX is
manually settable via PUT /api/stories/{id}. A later phase could close this
with a dedicated verdict-reporting AgentChannel method instead of parsing
free text.

go build/vet/test -race -count=1 all pass, full suite (20 packages).

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01V1moSNCJRcP6kykA4tyUSs

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