Make fast-path the only pipeline generation option

## Summary

Make the deterministic fast-path the only way to configure a GitHub Actions → AKS pipeline from the GitHub Pipeline wizard. Remove the full Copilot Coding Agent generation path. Keep the async "Copilot suggests improvements" follow-up as a default-on checkbox modifier. Block repos without a Dockerfile instead of degrading to the agent path.

## Motivation

The full Copilot Coding Agent path is a large, slow (~15–40 min) flow with substantial code surface (`AgentSetupReview`, `agentTemplates`, `useAgentPRDiscovery`, `useAgentWorkflowProgress`, the setup-PR / agent-task / generated-PR machinery, several extra deployment states, and a parallel set of wizard step labels). The deterministic fast-path is faster, more reliable, and produces tested artifacts. If a user doesn't have a Dockerfile, they should add one externally — this app shouldn't try to generate it.

## Proposed flow

Single set of wizard steps: **Connect Source → Configure → Deploy**.

- **Connect Source** — unchanged.
- **Configure** — covers ACR + workload identity setup. At `ReadyForSetup`:
- If a Dockerfile is found: render `DockerfileConfirmation` plus a checkbox **"Let GitHub Copilot suggest improvements (opens a follow-up PR)"** (default on). Primary button: **"Deploy"**.
- If no Dockerfile: render a new `NoDockerfileBlock` with guidance, doc links, and a "Choose a different repo" button.
- **Deploy** — fast-path states only.

## Code impact (high level)

**Delete:** `PathSelectionStep`, `AgentSetupReview`, `ReviewAndMergeStep`, `agentTemplates`, `useAgentPRDiscovery`, `useAgentWorkflowProgress`, agent-only deployment states, `AGENT_PATH_STEPS`, agent-path branches in `getWizardStep` and `useGitHubPipelineOrchestration`.

**Add:** `NoDockerfileBlock.tsx` and a "Suggest improvements" checkbox inline next to `DockerfileConfirmation`.

**Simplify:** `GitHubPipelineWizard` connector replaces `pathChoice` with a `withAsyncAi` boolean. `GitHubPipelineWizardPure` switch arms drop in half.

**Keep:** all of `fastPathOrchestration`, `fastPathTemplates`, `useFastPathOrchestration`, `useFastPathPipelineState`, `triggerAsyncAgentReview`, `useDockerfileDiscovery`, `DockerfileConfirmation`, ACR + workload identity setup.

## Migration

`pipelineStorage.load` migrates any persisted agent-path state (`SetupPRCreating`, `SetupPRAwaitingMerge`, `AgentTaskCreating`, `AgentRunning`, `GeneratedPRAwaitingMerge`, `PipelineConfigured`) back to `Configured` and clears the entry, so users mid-flight on the agent path land back at step 1 after upgrading.

## Out of scope

- Generating Dockerfiles ourselves or via Copilot handoff
- Re-detecting a newly added Dockerfile without restarting the wizard
- Changes to the deterministic templates
- Any Deploy Wizard changes outside the GitHub pipeline path