feat: support Jinja2 template expressions in agent model field

[Shazwazza]

Add Jinja2 template rendering for the model: field in agent definitions, enabling dynamic model selection via workflow inputs.

Previously only prompt, system_prompt, and when fields supported Jinja2 expressions. This extends that to model:, allowing workflows to parameterize which model each agent uses:

workflow:
inputs:
selected_model:
type: string
default: "claude-sonnet-4.6"

agents:
- name: reviewer
model: "{{ workflow.input.selected_model }}"
prompt: "Review this code..."

Changes

• src/conductor/executor/agent.py — Render model field in AgentExecutor.execute() when {{ or {% is detected. Uses model_copy() to avoid mutating the original AgentDef.
• src/conductor/config/schema.py — Update AgentDef.model docstring to document template support.
• tests/test_executor/test_agent.py — Add 3 tests: template rendering, static passthrough, non-mutation of original.

Motivation

Multi-agent workflows (e.g., PR review with multiple reviewer perspectives) benefit from letting users choose models at invocation time rather than hardcoding them. This enables A/B testing across models, cost optimization, and user preference without forking workflow
definitions.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
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Additional details and impacted files

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