Implements GitHub issue microsoft#65 — allows the same agent to appear at multiple
stages in a workflow with different prompts, inputs, and outputs while
sharing model, tools, and system prompt configuration.

## Design

Agents can now declare a stages dict with named invocation points:

`yaml
agents:
  - name: vp_engineering
    model: gpt-4.1
    prompt: 'Set technical direction...'
    stages:
      review:
        prompt: 'Review your team''s output...'
        input: [ic.output]
`

Stages are expanded into synthetic AgentDef instances at config load time
(e.g., �p_engineering:default, �p_engineering:review), so the workflow
engine sees regular agents and requires minimal changes.

## Changes

**Schema & Config** (src/conductor/config/):
- New StageDef Pydantic model with optional overrides (prompt, input,
  output, routes, description)
- AgentDef.stages: dict[str, StageDef] | None field
- �xpander.py — stage expansion with name collision detection, route
  rewriting, and entry point rewriting
- Validator updated for stage-qualified input refs, for-each inline
  agent stages rejection, and parallel group stages rejection

**Engine** (src/conductor/engine/context.py):
- Dual-key storage: stage-qualified outputs stored under both
  �gent:stage and �gent (latest) keys for backward compatibility
- _build_stages_dict() for Jinja2 access via stages.agent.stage.output
- Stages dict injected into all three context modes

**Tests** (42 new tests):
- 	est_expander.py — 34 tests covering StageDef schema, expansion,
  edge cases, name collisions, for-each validation
- 	est_staged_workflow.py — 8 integration tests covering VP→IC→VP:review
  execution, loop-back, backward compatibility, context storage

**Example**: �xamples/staged-review.yaml — VP→IC→VP:review pattern

Closes microsoft#65

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>