Add interactive prompt for migrating from APIOps Toolkit to APIOps CLI

## Problem

Customers currently using the [APIOps Toolkit](https://github.com/Azure/apiops) have no guided path to migrate to APIOps CLI. Migration requires manually converting configuration files, understanding schema differences, identifying missing properties, and rewriting CI/CD pipelines — all of which is error-prone and requires deep knowledge of both tools.

## Context — Migration Challenges

1. **Configuration file format differences** — Toolkit and CLI use different file structures and schemas; manual conversion is tedious and easy to get wrong
2. **Missing properties** — The CLI may require properties that don't exist in Toolkit configuration files, and these need to be discovered from the running APIM environment
3. **Override files** — Newly fetched properties need to be intelligently categorized into override files (e.g., environment-specific values, secrets, backend URLs)
4. **Pipeline migration** — Existing CI/CD pipelines reference Toolkit-specific steps/tasks that must be replaced with APIOps CLI equivalents

## Proposed Solution

Add a new interactive prompt (e.g., `apiops migrate` or similar) that guides users through a full migration from APIOps Toolkit to APIOps CLI. The prompt should:

### 1. Detect & Convert Existing Toolkit Files
- Scan the repository for existing APIOps Toolkit configuration and artifact files
- Map Toolkit file formats and schemas to their APIOps CLI equivalents
- Convert configuration files (extraction config, publisher config, etc.) to the CLI format
- Report any values that cannot be automatically mapped and prompt the user for input

### 2. Query Running APIM Environment for Missing Properties
- Connect to the user's Azure APIM instance to fetch any properties or resources that exist in the live environment but are absent from the Toolkit files
- Compare live state against the converted files to identify gaps
- Pull down missing artifacts as needed to produce a complete CLI-compatible artifact set

### 3. Smart Override File Generation
- For any additional properties pulled from the live environment, make intelligent decisions about what belongs in override files vs. base configuration
- Identify environment-specific values (e.g., backend URLs, named value secrets, diagnostic settings) and route them to the appropriate per-environment override files
- Use `{#[TOKEN_NAME]#}` placeholder syntax for secrets (per #117)
- Present recommendations to the user for review and confirmation before writing

### 4. Pipeline Migration
- Detect existing CI/CD pipeline files (GitHub Actions workflows, Azure DevOps pipelines)
- Identify and remove APIOps Toolkit-specific steps/tasks
- Replace them with the equivalent APIOps CLI steps
- Preserve any custom steps or configuration the user has added around the Toolkit steps
- Present a diff or summary of pipeline changes for user confirmation

### 5. Interactive & Best-Effort
- The prompt should be conversational — guide the user through each migration phase rather than requiring upfront knowledge
- Make sensible defaults and best-effort suggestions throughout
- Allow the user to review and confirm before writing any files
- Provide a summary report at the end listing what was migrated, what was added, and any manual steps remaining

## Acceptance Criteria

- [ ] New interactive prompt is available (e.g., `apiops migrate`)
- [ ] Detects existing APIOps Toolkit configuration and artifact files in the repo
- [ ] Converts Toolkit configuration files to APIOps CLI format
- [ ] Connects to a running APIM environment to fetch missing properties/artifacts
- [ ] Intelligently assigns newly fetched properties to base config vs. override files
- [ ] Generates per-environment override files with `{#[TOKEN_NAME]#}` placeholder syntax for secrets
- [ ] Detects and migrates CI/CD pipelines (GitHub Actions and/or Azure DevOps)
- [ ] Removes Toolkit-specific pipeline steps and replaces with CLI equivalents
- [ ] Preserves custom pipeline steps during migration
- [ ] Provides interactive review and confirmation at each phase
- [ ] Produces a summary report of migration actions taken and any remaining manual steps
- [ ] Documentation explains how to run the migration prompt and what to expect