Extract load lifecycle orchestration

[gadflysu]

Goal

Extract load lifecycle orchestration from main.go into a shared boundary so list mode, interactive mode, and the future debug-load performance hook can reuse source loading behavior without copying logic.

Current problem

main.go currently owns two load paths:

• list mode waits for a complete session list before rendering rows;
• interactive mode starts the picker immediately and feeds it streaming batches.

The source loaders are partly reused, but orchestration concerns are split across main.go: source selection, source concurrency, date filtering, partial failure handling, complete-load result construction, stream completion, and batch ownership.

Desired direction

• Add a dedicated load lifecycle boundary.
• Keep complete loading and streaming loading as separate caller-facing paths.
• Move the streaming batch contract to the load boundary so picker consumes the loader-owned batch type directly.
• Preserve current behavior for list mode and interactive mode.
• Do not add new CLI flags in this issue.

Non-goals

• Do not implement --debug-load; that belongs to Add debug-load performance hook #59.
• Do not change source parsing behavior.
• Do not change list output formatting.
• Do not change interactive key handling or picker rendering.
• Do not add benchmark tooling.

Plan

See docs/agent/plan-issue-61-load-lifecycle-refactor.md.