feat: enforce schema-backed agent outputs

[eshulman2]

Summary

• add typed Pydantic response contracts for data-shaped agent stages
• preserve the Deep Agent tool loop while preferring provider-native structured output
• retry once with LangChain structured-output tool strategy when native output is unavailable or invalid
• route model selection through an explicit structured_output capability
• migrate epic/task generation and triage stages away from ad hoc model-text parsing
• document backend behavior, limitations, and narrative-output exclusions

Stack

• Base: refactor: remove Phase 8 compatibility paths #330 (phase8-compatibility-removal)
• This PR should be retargeted to main after refactor: remove Phase 8 compatibility paths #330 merges.

Verification

• 79 passed in the focused structured-output/provider/workflow suite
• ruff check passes for every changed Python file
• git diff --check passes
• full suite: 3092 passed, 39 skipped; 13 pre-existing failures remain in unchanged tests/integration/orchestrator/test_workflow_execution.py, which imports the legacy forge.orchestrator modules already removed by the Phase 8 base

Closes #252