Prune dead tests for removed runners and node types

[lambdabaa]

Parent: #1301

What

After the runner and node type pruning sub-issues are complete, clean up ~1,960 lines of dead test code that tested the removed functionality.

Scope

• Remove bob runner tests
• Remove codex runner tests
• Remove opencode runner tests
• Remove tests for SubgraphForkNode, SelectionNode, LLMNode
• Remove tests for background node execution
• Remove tests for parallel_improve_workflow (if moved)

Dependencies

Blocked by:

• Prune dead runners: bob, codex, opencode #1326 (Prune dead runners)
• Prune unused node types (above)
• Prune background node support (above)

Validation

• pytest -v passes with no collection errors
• Coverage does not drop on remaining code