feat: spawn_agents_on_csv — batch fan-out for audit/review workflows

[justrach]

Parent: #257 (Phase 2)

What

Batch fan-out tool inspired by Codex's multi-agent CSV pattern. Given a CSV of work items, spawn one worker agent per row, wait for all to finish, export combined results.

Use cases:

• Review one file/package/service per row
• Check a list of incidents, PRs, or migration targets
• Generate structured summaries for many similar inputs

Accepts: csv_path, instruction (with {column_name} placeholders), id_column, output_schema, output_csv_path, max_concurrency, max_runtime_seconds.

Each worker calls report_agent_job_result exactly once. Failed rows marked in exported CSV.

Depends on

• feat: provider dispatch — backend registry (claude/codex/sdk/local) #262 (route through agent_sdk)
• feat: role registry — named agent specs via config.toml + custom .md roles #260 (role registry)

[justrach]

Blocked by: #260.