TUI v2 3.1: Extract start-cluster helper for TUI

[tomdps]

Summary

Implement Issue 3.1 from the TUI v2 plan: extract a minimal reusable "start cluster" helper for the TUI so it can launch runs without duplicating CLI logic.

Scope

• Add a small adapter module (JS) that encapsulates:
  • explicit input construction (plain text vs /issue ref)
  • provider override resolution
  • config loading (resolveConfigPath, loadClusterConfig)
  • calling orchestrator.start(...)
• TUI uses this helper rather than duplicating CLI logic.

Non-scope

• No changes to CLI zeroshot run behavior.
• No changes to launcher UI or /issue command wiring (Issues 3.2/3.3).

Implementation Plan

1. Identify the minimal CLI path for starting a cluster and extract reusable helper(s).
2. Implement startClusterFromText and startClusterFromIssue wrappers (or similar) in a new module (e.g., src/tui/services/start-cluster.js).
3. Ensure helper accepts provider override + config path and passes correct options into orchestrator.start(...).
4. Add unit tests for helper inputs/outputs with mocked orchestrator and config loaders.

Tests

• Unit tests for helper functions (text + issue inputs).
• Ensure provider override is forwarded to orchestrator options.

Acceptance Criteria

• zeroshot run behavior unchanged.
• New helper can be unit-tested independently.
• TUI can call helper without duplicating CLI logic.

Dependencies

• Depends on Issue 2.2/2.3 (command system scaffolded).

References

• docs/tui-v2/IMPLEMENTATION_PLAN.md (Issue 3.1)
• docs/tui-v2/PRD.md (launcher and issue flows)

[tomdps]

Implemented in PR #188