azd tool: track alpha feature flag lifecycle and graduation

## Summary

The `azd tool` command group and its associated middleware (first-run tool check, background update check) are gated behind an alpha feature flag (`tool`). This issue tracks the feature flag lifecycle through alpha, beta, and GA stages.

## Current State

- **PR**: #8111
- **Feature flag**: `alpha.tool`
- **Default**: Disabled (commands not registered, middleware skipped)
- **Enable via**: `azd config set alpha.tool on` or `AZD_ALPHA_ENABLE_TOOL=true`

## Gating Points

1. **Command registration** (`cmd/root.go`) — `toolActions()` only called when flag is enabled
2. **First-run middleware** (`cmd/middleware/tool_first_run.go`) — early return when disabled
3. **Update-check middleware** (`cmd/middleware/tool_update_check.go`) — early return when disabled

## Graduation Criteria

- [ ] Alpha testing complete with internal users
- [ ] No critical bugs reported during alpha
- [ ] Tool manifest validated across Windows, macOS, Linux
- [ ] Promote to beta feature flag
- [ ] GA: remove feature flag, enable by default

## Related

- Epic: #7845
- Implementation PR: #8111